Inside the Kriterion cinema in central Amsterdam on a sodden summer evening, a birthday party is getting under way. The music is thudding out, skinny-jeaned students are edging their way on to the dancefloor, and the bar staff are run off their fashionably attired feet. Only one thing is peculiar about this event. The entity whose birthday is being celebrated tonight isn't a person. It's a typeface.

People have been pouring into the building all evening to attend the Dutch premiere of a biopic with a difference, a documentary about the life of a font, Helvetica, which celebrates its 50th anniversary this year. You may not know the name, but you'll almost certainly know the face: Helvetica, the typographic identity of Orange and Muji, of British Gas and US Mail, of United Van Lines as well as the United Nations, is probably the most popular font in existence. Even so, isn't a special birthday bash in its honour a little, well, extreme?

The film's creator and director Gary Hustwit, a small-set, close-cropped 42-year-old, smiles at my incredulity. "Oh, that was nothing," he grins when we meet for lunch the next day. "In Zurich, which was the European premiere, they brought out a huge cake and 800 people sang happy birthday." Really? "Oh, yeah," he says, chewing thoughtfully on his burger. "And when we showed the film in Philadelphia there was a giant H made out of Swiss cheese."

If Hustwit looks chuffed, it's easy to understand why. His first attempt at a full-length documentary, shot on a credit-card budget and made up of interviews with designers and typographers, has somehow become a global phenomenon. Since March, when the film premiered in Austin, Texas, it has been shown in cities from Auckland to Vancouver, Cologne to Santa Fe, and new dates are being added almost daily. Released in time for Helvetica's 50th anniversary, it's been screened at Zurich's Helvetica50 celebrations and also at a commemorative exhibition at the Museum of Modern Art in New York. The film will hit Britain at Oxford's Britdoc festival later this month, with a three-week run at London's ICA slated for early September.

On the face of it, such demand seems hard to explain. Typography, the art of crafting letters, numerals and symbols and arranging them in communicable patterns, is among the more recondite and poorly understood aspects of design. Most of us comprehend what it is architects and engineers do each day, just as we're used to grappling with the applied design that rules our daily lives, whether it's the shape of a kettle or the operation of a photocopier. But about typographic design, whether good or bad, we're clueless. Design a building and it's headline news. Design some text and barely anyone notices.

For all that, typography is ubiquitous. We require text, text requires form, form requires design. Since the invention of the printing press, the stuff has been everywhere. The idea for the film came, Hustwit says, when he was wandering the streets of New York, observing the way urban and typographic design collide and collude. "I was walking around, looking at the type and the way people interacted with it, and I just saw the whole film in my head. I wanted to do a documentary about graphic design, and I thought - well, Helvetica is the most ubiquitous typeface in our lives, why not that?"

In one of the film's most brilliant sequences, one of Hustwit's interviewees, the writer Lars Müller, takes a similar stroll around London, a city that, it turns out, is nearly as full of Helvetica as New York. Pointing gleefully at traffic signs, shopfronts and notices that "Posters Will Be Prosecuted," Müller pays tribute to what elsewhere he has called "the shift worker and solo entertainer of typefaces", the font that moonlights on the glossiest of corporate identities as well as the scummiest of homemade signs.

If you can't think what Helvetica looks like, glance around you: chances are it's somewhere in your field of vision, whether whistling past on the television news or staring out at you from the local dry cleaners. (Though no longer does it star on the pages of this newspaper - after 17 years' service, the font was retired in 2005, when the Berliner format was launched, and replaced by Guardian Egyptian.) Nor is Helvetica confined to the western alphabet: a Cyrillic version now exists, as do variants in Greek and Chinese. It is, I am slowly beginning to comprehend, the world's font.

For all that, and as the film reveals, Helvetica is indelibly associated with the country whose Latin name it pays tribute to: Switzerland, where a new movement in graphic design first took root in the 1950s. Energised by advances in typesetting technology, Swiss designers were hungry for typefaces embodying the principles of modernism - clean, crisp, neutral - and type foundries reciprocated with a wave of fresh designs. One of the first was Univers, an austere but elegant sans-serif face drawn up by Adrian Frutiger that appeared in 1956. It became an instant classic, its 14 different weights, ultrathin to über-bold, painstakingly numbered to offer technicians greater flexibility.

Univers was followed just a year later by a rival typeface designed by Max Miedinger and Eduard Hoffman of the Haas foundry in Münchenstein - the working drawings for which, annotated with Hoffman's pencil adjustments, appear in the film. Their new font was initially christened Neue Haas Grotesk but rapidly renamed Helvetica - a last-minute publicity decision that had far-reaching ramifications. Marketed as a symbol of cutting-edge Swiss technology, Helvetica went global almost at once.

One man at the heart of this revolution was Massimo Vignelli, the Italian-American designer who used Helvetica for an iconic American Airlines logo in 1966, distilling the company's forward-looking philosophy so brilliantly that it has remained unchanged ever since. Hustwit managed to interview Vignelli, and like the other typographers and technicians he spoke to, found him passionate about the social significance of his craft. "The life of a designer is a life of fight, fight against ugliness," Vignelli exclaims at one point in the film, "just like a doctor fights against disease." Small wonder that when he came to develop a new identity for the signage of New York's ailing subway system in the early 1970s, Vignelli reached yet again for Helvetica - this time stark white text on an unyielding black background, iconography as bold and brash as the city itself.

Part of the point of making a documentary about all this, Hustwit explains, is to put faces to the names of great designers such as Vignelli whose work surrounds us every day but whose identities remain more or less unknown. "When I started this project," he says, "I couldn't believe that a film like this didn't exist already, because these people are gods and goddesses. What they do is more than just logos and corporate branding - they design the type that we read every day in newspapers and magazines, onscreen and on television. Fonts don't just appear out of Microsoft Word: there are human beings and huge stories behind them."

Microsoft Word and the revolution in desktop publishing are, of course, part of the reason that audiences who wouldn't otherwise know their glyphs from their descenders are flocking to see a documentary devoted to the history of typography. Graphic design is no longer the province of specialists, but available to anyone with a word processor. "All these things that were the realm of professional designers 10 years ago are now being done by eight-year-olds," says Hustwit. "The more people are exposed to graphic design, the more they appreciate it."

· Helvetica is showing at BritDoc, Keble College, Oxford, July 25-27 (details: britdoc.org); and at the ICA, London, from September 7 (details: ica.org.uk/020-7930 3647). 50 Years of Helvetica is at MoMA, New York until March 31 2008.