On 25 September 2007, a woman named Vicki Walker committed a type crime so calamitous that it cost her not only her job, but almost her sanity. Walker was working as an accountant in a New Zealand health agency, and there was an email to send. Regrettably, she ignored the only rule that everyone who has ever emailed knows: CAPITAL LETTERS LOOK LIKE YOU HATE SOMEONE AND ARE SHOUTING.

It was a Tuesday afternoon. Walker pressed "Send" on this instruction: TO ENSURE YOUR STAFF CLAIM IS PROCESSED AND PAID, PLEASE DO FOLLOW THE BELOW CHECKLIST.

Not the written word's finest hour in lots of ways, but hardly a sackable offence. The letters were in blue, and elsewhere her email contained bold black and red. She worked for ProCare in Auckland, a company which clearly placed great pride in knowing when and when not to hold down the Caps button, though it did not have an email etiquette guide at the time Vicki Walker splurged on upper case.

Upper and lower case? The term comes from the position of the loose metal or wooden letters laid in front of the traditional compositor's hands before they were used to form a word – the commonly used ones on an accessible lower level, the capitals above them, waiting their turn. Even with this distinction, the compositor would still have to "mind their ps and qs", so alike were they when each letter was dismantled from a block of type and then tossed back into the compartments of a tray.

The correct use of type varies over time. These days, corporate edicts are common, and memos come down from on high like tablets of stone: thou shalt use only on both internal and external communications. But who is to say that lower-case Arial from 1982 is preferable to the way we communicated in TRAJAN CAPITALS on the pediments of public buildings in ancient Rome? And how did our eyes switch from accepting one over the other, to the point where a thoughtless choice of capitals-all-the-way became a cause of headaches and dismissals?

Walker was sacked three months after her email was deemed to have caused "disharmony in the workplace", which would have been laughable had it not caused her so much distress. Twenty months later, after remortgaging her house and borrowing money from her sister to fight her case, Walker appealed successfully for unfair dismissal, and was awarded $17,000 (£10,000).

The etiquette of type informs our daily lives. Say you are designing a jacket for a new edition of Jane Austen's Pride and Prejudice. The book is out of copyright and so has cost you nothing, the beautiful jacket illustration of a secret garden has been done by a friend, and now all you have to do is find a suitable typeface for the title and author, and then the text inside. For the jacket type, conventional wisdom would be to choose something like Didot, which first appeared at around the time Austen was writing and looks very classy with its extreme range of fine and stronger lines, especially in italics. This font will fit right in, and will sell books to people who like classic editions. But if you wanted to reach a different market, the sort who might read Kate Atkinson or Sebastian Faulks, you may opt for something less fusty, perhaps Ambroise Light, which, like Didot, has a stylish French pedigree.

For the text of the book, you might consider a digital update of Bembo – perhaps Bembo Book? Originally cut from metal in the 1490s, this classic roman typeface retains a consistent readability. And it fits the overriding principle that typefaces should mostly pass unrecognised in daily life; that they should inform but not alarm. A font on a book jacket should merely pull you in; once it has created the desired atmosphere it does well to slink away, like the host at a party.

There are exceptions, of course, and a brilliant one is John Gray's bestseller, Men Are From Mars, Women Are From Venus, in which the designer Andrew Newman chose Arquitectura for the male lines and Centaur for the female ones. Arquitectura looks manly because it is tall, solid, slightly space-age, rooted and implacable. Centaur, despite its bullish name, looks like it has been written by hand, has thin and thick strokes, and is charming and elegant (obviously this is gross sexua l stereotyping, but Men Are From Mars, Women Are From Venus is pop-psychology).

This then is another rule: type can have gender. The understanding is that heavy bold jagged fonts are mostly male (try Colossalis), and whimsical, lighter curly fonts are mostly female (perhaps ITC Brioso Pro Italic Display from the Adobe Wedding Collection). You can subvert this form, but never the automatic associations that type infers. It's the same with colour: you see a baby dressed in pink – that's a girl. Type has us conditioned from birth, and it has taken more than 500 years to begin to shake it free.

Fonts were once known as founts. Fonts and founts weren't the same as typefaces, and typefaces weren't the same as type. In Europe the transition from fount to font was essentially complete by the 1970s, a grudging acceptance of the Americanisation of the word. The two were used interchangeably as early as the 1920s, although some whiskered English traditionalists will still insist on "fount" in an elitist way, in the hope that it will stretch their authenticity all the way back to Caxton, the great British printer of Chaucer. But most people have stopped caring. There are more important things to worry about, such as what the word actually means.

In the days when type was set by hand, a font was a complete set of letters of a typeface in one particular size and style – every different a, b and c in upper and lower case, each pound or dollar sign and punctuation mark. There would be many duplicates, the exact amount dependent on their common usage, but always more Es than Js. The word is derived from "fund", the fund (amount) of type from which the letters are selected. These days a font refers simply to a particular typeface, which may have 10 or 20 fonts, each weight and style on the page a little different. But in common parlance we use font and typeface interchangeably, and there are worse sins.

Definitions should not cloud our appreciation of type, but some classifications can be useful in understanding the subject's history and usage. Just as it is entirely possible to have a pleasant afternoon in a gallery with no knowledge of art theory or an artist's place in the firmament, one can wander around the streets admiring typefaces on signs and shops with not a care for their history. But it may increase our love of them if we know who made them, and what they were aiming for. And for this we need to define a few words in the typographic language.

Bodoni and Baskerville are both serif fonts, while Helvetica is a sans serif. The difference lies at the feet or tips of the letters, with a serif typeface carrying a finishing stroke often appearing to ground the letter on the page. This could be the base of an E,M,N or P, but it could also be the left-upper flick of an r or the roof of a k. It makes the letters look traditional, square, honest and carved – and their lineage can be traced back as far as the Roman emperor Trajan, whose column in Rome, completed in 113, bears an inscription in his honour and serves as the most influential piece of anonymous stone carving in 2,000 years.

Sans serif faces may appear less formal and more contemporary, but they can be as redolent of tradition as a brass band. Many have a very classical and Roman form – indeed sans serif lettering existed in the Ancient World – and when they appeared on buildings in fascist Italy between the wars they fitted right in, as if they had been there for centuries. They are durable and may be monumental, and while Futura, Helvetica and Gill Sans are the best known, there are countless numbers in our daily lives. The oldest sans serif type is probably Caslon Egyptian from 1816 and through the 19th century they became popular mainly as display fonts, for use on posters.

However, in the following century sans serif type took on a very different character, as a new generation of designers fused the Roman and display traditions with modern style. Nothing looked so good stuck on the side of a new machine, or, as with Edward Johnston's typeface, on the London Underground. The roots of this new sans serif lay in Germany, in a font known as Akzidenz Grotesk, released in 1898. But it was given a new life in Britain by Johnston and by Eric Gill's Gill Sans, and by others in Germany, Holland and – most notably – in post-war Switzerland, where Univers and Helvetica arose to spearhead modernism's spread across the world. So we'd best think of the type now as European.

Because there are so many typefaces, there have been many attempts to classify them into definable groups. But type is a living element, and it will resist absolute categorisation until it is worn thin; a good single letter in a vivid typeface has enough energy in itself to leap free from any box.

Within each typeface, a single letter has its own geography. This requires an exact language that is charming and unforgiving, jargon which began with the punchcutter from the 15th century and has resisted all attempts at digital corruption. Counters are the enclosed or semi-enclosed areas of a letter, within an o, b or n, for example; while the bowl is the curved shape of the g, b, etc; and stems are the main constructional elements – which may be thick or thin depending on design.

A bracketed serif has a curved element like a tree trunk, an unbracketed one is a straight line, while a wedge serif falls at a geometric angle. The x-height of a letter is the distance between the base line (the line in an exercise book) and the mean line (the top of a lower-case letter); an ascender rises above the mean line, a descender below the base line.

Some type vocabulary has an internal beauty of its own (or it did when all type was metal). Much of this is anthropomorphic, treating letters as living life-forms: the whole character is known as the body, the blank space below the raised letter is the beard, the flat side of the metal type is the shoulder, while the whole raised letterform is the face. Traditionally, a ligature has meant a light linking flourish between two letters that are joined together (such as fl or ae, which require less white space between them than if the letters were used on their own). These days, commonly, a ligature (a feature of both serif and sans serif faces) refers to the two letters themselves, used as if they were one.

And then there is maths. The point size can be used both as a unit of measuring type and the space between it. For regular newspaper and book text, 8pt to 12pt usually satisfies. There are 72pts to an inch. 1pt is 0.013833 inches.

Typographers group them in picas: 12pts to a pica and 6 picas to an inch. There have been many historical and national variations, and metal and digital measures differ slightly, but today we almost have an international standard: in the US, 1pt = 0.351mm; in Europe 1pt = 0.376mm.

But the maths, geography and vocabulary of type should never obscure the most basic fact of all: regular or italics, light or bold, upper or lower case – the fonts that work best are the ones that allow us to read without ruining our eyes.

In a wood, somewhere in England, rifles in hand,

"You have been watching:

Arthur Lowe (proud, pompous walk)

John Le Mesurier (leafy camouflaged helmet, looking nervous)

Clive Dunn (brave gaze, cold steel)

John Laurie (anxious, doomed)

James Beck (crafty draw on cheeky fag)

Arnold Ridley (may need to be excused)

Ian Lavender (blue scarf, mum's insistence)."

This is the closing sequence to Dad's Army, Britain's much-loved TV comedy about the second world war, produced in the late 1960s/early 1970s and repeated ever since. The actors' credits are in Cooper Black, which sells not only things we now consider to be retro and classic, such as Kickers or Spacehoppers, but also anything intended to be warm, fuzzy, homely, reliable and reassuring, like easyJet.

The lettering on the side of planes had rarely implied fun ("We're one of you! climb aboard!") before easyJet tried it, and so strong is this typographic branding that no one has successfully imitated it. (Although the budget airline's chief rival, Ryanair, once used Arial Extra Bold – attracted perhaps by its name – before moving to a proprietary font.)

EasyJet's branding soon extended to the easyGroup's other products, and was discussed in the company's mission statement:

Our visual identity, known as the "Getup", is an essential part of the easyJet Brand Licence and is cast in stone! It is defined as: a) white lettering on an orange background (Pantone 021c on glossy print materials; on other surfaces the nearest practicable equivalent), and b) in Cooper Black font (not bold, italics, outline nor underlined), the word "easy" in lower case, followed (without space) by another word …

Cooper Black was a good find. It is rare for a new company to select a pre-digital unmodernised classic face from the shelf and not revive or tweak it in some way, but here was an exception. Like so many fonts that have stuck, it was designed in the 1920s, and became instantly popular. Oswald Bruce Cooper, a former Chicago advertising man, was commissioned by the foundry Barnhart Brothers & Spindler to make something that they could sell to advertisers (and something that looked suspiciously similar to Pabst Extra Bold, designed several years before by Frederic W Goudy for the American brewing firm).

Its success soon allayed Cooper's fear that he would only achieve "a tiresome effect from the too frequent repetition of the same quirk and curve". In fact he achieved something spectacular – a serif face that looked like a sans serif. Cooper Black is the sort of font the oils in a lava lamp would form if smashed to the floor. Its creator believed it ideal "for far-sighted printers with near-sighted customers". There are little nicks at the tops and bases of letters, and they give the font a solid flat weight on a page; without them, the type would always have been appearing to roll away. For a font with such a thickset look, it retains a remarkably unthreatening demeanour. Partly this is due to its stout and pudgy descenders, its large lower-case letters in relation to its capitals, and the limited white peering through the counters of the a, b, d, e and g. It is usually employed quite bunched up, for excessive spacing between letters would make it break up very fast, confusing the eye.

Cooper Black looks best from afar, as easyJet recognised. Before then, its most famous appearance was probably on the classic Beach Boys album Pet Sounds. Like many record covers of the time, this printed each song title on the front of the sleeve – above the photograph of the band feeding goats at the zoo. Cooper Black for the band name and title is iconic, not least because the letters are touching, and reflect Robert Indiana's then very much in-vogue "Love" logo. But its weakness as a text font is immediately clear. "Wouldn't It Be Nice/You Still Believe in Me" , the line runs, before our brain unscrambles the rest of the offering, "God Only Knows", "Sloop John B" and the others. The 12in record sleeve gets away with it; the CD cover is very hard work.

The graphics people responsible for the Dad's Army credits knew they'd have problems once the big stars with the big lettering left our screens; the show's lesser characters are credited for a briefer period, and with no pictures: "Featuring Philip Madoc as the U-Boat Captain… with Bill Pertwee as Chief Warden Hodges…" Before 42in widescreens, viewers couldn't possibly handle all of this in small Cooper Black, so only the actors' names appear in this font, and the parts they're playing are in something resembling Helvetica.

This is one difference between legibility and readability: at small sizes, Cooper Black is legible but not very readable. But some type is meant to be seen rather than read (a type designer once compared this attribute to a dress designed to look great on the catwalk but provide no protection against the elements). Font-as-couture is a common analogy. Adrian Frutiger, designer of one of the most popular modern fonts, Univers, had another: "The work of a type designer is just like that of a dressmaker," he noted. "Clothing the constant, human form." Or as the book designer Alan Fletcher put it, "a typeface is an alphabet in a straitjacket".

Fonts, like life, are governed by rules. It's not necessarily a bad thing, being told to sit up straight and not covet thy neighbour's ox, and we'd all be lost without i before e except after c. But to what extent do rules stifle individuality and creativity? What happens to the minds of a million first-year art students when faced with the task of designing a great new typeface? They too are contained by parameters, creativity blocked in like a palette knife shaping wet cement.

The most instinctive designers (the true artists) will know what works and what to discard. But the novice is hampered by the weight of history and the dead grey hand of the instruction manual. For many of the rules of type are really rules of typography, bound up not just with their appearance but with their use on a page; useful on their own, they can be destructive in combination.

In mid-October 2004, the highly regarded printer, designer and writer Sebastian Carter delivered the Beatrice Warde Memorial Lecture at the St Bride Institute, as part of a three-day symposium entitled "Bad Type".

This was the rulebook in reverse, or white on black. Carter began by running through several notable instruction manuals on type and typography, which had "a way of imposing solutions and limiting choices". He discussed the value of the jobs undertaken by small presses and extolled the worth of ephemera. Carter himself ran the small Rampant Lions Press in Cambridge, where he (and his father, Will, before him) produced exquisite work. But he also championed the not-such-a-great-job, the pieces of design and printing that didn't turn out to be beautiful or clear, merely interesting. He illustrated his talk with some items that were "pretty cruddy", and suggested that these, too, had a place in our world. "I would not want to live in a world of exclusively good design at the bus-ticket level," he said.

Carter was preceded onto the platform by Nigel Bents, a senior lecturer at Chelsea College of Art and Design, who pronounced that he had had enough of perfect type, wrought according to perfect rules. In its place he proposed a love letter to disaster. "What we need is a manifesto," he told his audience of designers, "set diagonally and vertically, all script caps with soft shadows, outlined and underlined, with poor punctuation and hundreds of hyphens, stretched to the edge and cropped at the sides, printed in yellow on day-glo paper, trimmed badly and poorly presented."

Thus armed, "the designers of tomorrow will not look back; we give them the chance to fail abjectly and completely; they're all in the typographic gutter and some of them are looking at their scars." The result, of course, would bring forth more failure, but also types of originality and brilliance. "We could become a nation of typographic genii through a litany of design atrocities," Bents reasoned.

But is that what we've got now? Does the type you see around you enhance your day? A survey of manifestos and guide books suggests that our hands can only be held so much, and for the rest we must trust to inspiration. The only intractable, invincible basic rules of good type? Make it interesting, make it beautiful, and bring out both its humanity and its soul. Make it tasteful and witty and apt. And readable.

Or change your name to Neville Brody. If you were Neville Brody you could join a London-based magazine called the Face in 1981 and transform its rather predictable design to such a degree that your impact would be felt not just on the look of magazines but on books, music, and many aspects of commercial product design for decades to come. If you were Neville Brody, you could set up a design business called Research Studios with offices in five countries and rework the retail look of high-end fashion and perfume brands, and take up the post of head of communication, art and design at the Royal College of Art. You could give your typefaces monumental names like Typeface Four or Typeface Six and you could continue to wear a ponytail long after everyone else in the design world had been ridiculed into chopping theirs off.

Brody studied at the London College of Printing in the late 1970s, where he fell under the spell of punk and the possibilities of non-conformity, and where he was almost thrown out for designing a stamp with the Queen's head placed sideways. This was not just the punk of the Sex Pistols and their designer Jamie Reid's scissorly disrespect for old typography, but also the anarchy of his hero Alexander Rodchenko, who suggested that creativity was simply the force that people who made rules disapproved of.

Brody found his first expression in record sleeves, learning from Barney Bubbles at Stiff Records and Al McDowell at the design studio, Rocking Russian. His work at the Face pushed, pulled, squeezed and bent type as it explored the edge between structure and legibility, and it sat well alongside the magazine's unusual use of Futura, Gill Sans Bold Condensed and Albertus. The wild geometry of his designs originated in the traditional way (drawing and cutting out shapes, working with Letraset and copiers) but it was the boldness of his display that shook people – text occupying an entire page, the overlapping of type and a crashing of styles, the way the word "contents" would gradually disintegrate across five monthly issues. He disliked the restrictions suggested by Beatrice Warde and Jan Tschichold, and he would try anything to shake off their suffocating influence.

By the time Brody got to work on Arena magazine in 1986 and the Victoria and Albert Museum displayed his typography two years later, his daring visual jokes and eagerness to confound had entered the consciousness of graphic design students throughout the world. His embrace of digital possibilities continued to produce stunning fonts – from his Futura-style Insignia and fluid Blur in the early 1990s to his hard-edged Peace in 2009 – and they were often accompanied (in Brody's mind at least) by a weighty emotional or political message.

To hear Brody address his adoring students today – at London's Design Museum in the shadow of London Bridge – is to realise how little his world view has changed. At 52 he still carries a fizzing resentment towards conformity, and expresses the hope that the economic crisis will usher forth a comparable cultural rebellion to the one that fired him 30 years before. "Where is the language of protest now?" he asks. "We have been led to believe that culture was only there as a financial opportunity." He talks about danger and originality as the screen behind him flashes images from Man Ray and the Factory Records designer Peter Saville.

Brody conducts a swift slide show of his recent hits: the titles for the Michael Mann film Public Enemies, recent designs for Wallpaper* and the typography magazine Fuse, a wall of surveillance cameras for his Freedom Space downstairs in the London Design Museum's Super Contemporary show. He pauses at an image of the Times from the 19th century, comparing it to his own redesign of the paper in 2006. He said he intended to give it greater clarity and energy, and one of his tools was new type – 8.5pt Times Classic for the main text and Obama's campaign face Gotham for the more compressed sans serif headlines.

The paper was following a trend: newspapers looking more like their web pages. The previous year the Guardian had also changed its type, from a mix of Helvetica, Miller and Garamond to its own Guardian Egyptian, a versatile and gentle font family comprising 96 variants designed by Paul Barnes and Christian Schwartz to accompany the paper's move to a smaller format.

The key, Brody said, was "to change a newspaper entirely, but to make sure no one noticed. Our main focus was on articulation, so the layout people could use each page as theatre. When we first showed it to focus groups they didn't notice it had changed, but when we told them it had changed, they hated it."

Before he adjourned for a drink, Brody told his students that he was concerned about the genericism of our culture. "Everywhere you go has similar spaces and signs," he said. "As designers we are complicit in this – we have to look for new ways forward. It's all about words that we don't use any more, like revolution and progress." But there were limitations to his vision, an ambition sapped by studio bills, staff wages and the need to think deeply about branding possibilities for multinational clients pushing luxury products.

As for the Brody brand itself, the iconoclast still finds his purest refuge in type. His font designs of 2010 were called Buffalo and Popaganda, huge and beautiful architectural slabs of ink that clamber over each other in the fashionable magazines, always challenging and arresting, never content just to sit there and tell a story.

Extracted from Just My Type: A book about fonts, by Simon Garfield (Profile Books, £14.99)