Subs, or subeditors, can get a lot of stick on this site, and many people aren't really sure what we're here for – except perhaps to mess up copy or write boring/wrong/sensational headlines. But we are basically the last line of defence – whether we save a writer from a legal suit, looking daft or being simply unintelligible – and our furniture (headlines, standfirsts – also known as subheadings – and captions) can be decisive in whether a story is read or ignored.

I was taught to subedit by a wonderful bunch of male, middle-aged reprobates who worked on the news pages of a national newspaper, one of whom helpfully advised me that nobody liked subs. They certainly had their idiosyncrasies. Turning up to work I'd often find one lying on the floor attempting something almost but not quite entirely unlike yoga, a slight whiff of whisky in the air.

Sometimes there'd be someone singing folk tunes or shouting loudly about their wife. They somewhat resembled the eccentric daytime drinkers in your local pub. Except that an hour into their shift each man would have his nose to the keyboard, furiously bashing copy into shape, blasting off a flurry of punning headlines with the complexity of crossword clues and fielding shouts of last-minute alterations from editors across the room all night until deadline crashed down and everyone went to the pub.

The environment has changed somewhat since those heady days. Some papers have turned to outsourcing while others have been experimenting with doing away with subs completely.

I mainly work on the internet now, on the Comment desk, with a rather more diverse bunch of folk. We are more consistently busy – and 100% more sober, honest – but the core skills required aren't all that different. We still make sure copy is readable, rewriting where necessary, and check facts. We don't cut copy to fit, as you do on a newspaper, but sometimes a piece may need a heavy rewrite, and we can tread a fine line between ensuring copy is clear and annoying readers familiar with the topic by over-explaining niche references.

Working online, we choose and upload pictures and add keywords – to index a piece within the theguardian.com site – and links, trying to use primary sources where possible. But the big difference is the headlines. In a newspaper, headlines can be lyrical, imaginative, off the wall. They can retain an air of mystery. On the internet, a different approach is required.

In many online contexts – on a mobile device, for example – users will see only the headline at first, so it has to stand alone and make it very clear what the piece is about. Furthermore, Google and other search engines look mainly at the headline when indexing stories, although the text in the standfirst, caption and first paragraph are also relevant. So for a piece to be easily found, we need to include search terms as near the beginning of the headline as possible. The challenge is to still make it interesting and fun within these constraints. Sometimes we'll slip a pun in after we've got the keywords out of the way, or use the keywords as a kicker, or similar. The trick is to try, hopefully relatively seamlessly, to integrate the keywords into a coherent line. If you want to read more about, say, the late Galápagos tortoise Lonesome George, you're never going to find a story headlined "Slow but sure symbolic", which might have worked better on the pages of a newspaper. The Sun's famous headline "Super Caley go ballistic, Celtic are atrocious" would probably not have made it past the SEO (search engine optimisation) desk.

Another common complaint of subbing is that the wrong angle has been taken in the headline and standfirst. Often with a complex piece, getting across the thrust of the argument in a few words can be our most challenging task, and sometimes readers will disagree with our decision. There is an element of subjectivity here, which is one of the reasons a "revise" sub takes a look before the piece goes online.

Picture decisions can also irritate readers. We always prefer to use images if we can, and another challenge can be to find something relevant. Sometimes our choice can appear tangential, but it may present an opportunity to pull out a quote from the piece, as well as reinforce the keywords for search engines. There are exceptions – if a story is sensitive or philosophical, using stock images can be insensitive or irrelevant.

Whatever some may say, we're not trying to sensationalise, we're trying to make sure that people interested in the subject get to read what our writers have to say. Yes, we want our piece to be read by as many people as possible, not just in a greedy hunt for hits (although of course we want to be popular …), but also to make the whole process worthwhile.

We do, of course, make mistakes, and we rightly get pulled up for it. But go for cock-up rather than conspiracy. We try to correct them as soon as we can too – do keep flagging up errors if you see them, although subs can't read every thread in full, so if you spot something factually incorrect, please email the readers' editor.

I hoped when I first started the job that it wasn't true that nobody liked subs. I still have some faith, and I'm sure I'll see a comment at the end of a piece soon that says "not a single error, factual or grammatical, in this piece, it flows beautifully, the headline perfectly matches the copy and the caption sings". Go on, make a sub's day.