What do people usually wear to conferences? I'm going to my first conference in about a month, where I will be presenting a poster.

How do people usually dress for conferences? Do I need to dress more smartly than usual? Is formal dress (e.g. a suit) normal? Or does no-one really care?

I'm a PhD student, if that makes any difference, and both my university and the conference are in the UK. It's a biology conference.
It depends a lot on the event (some gatherings are more formal than others) and on your status (win a Nobel prize, and you can wear anything you damn well want), but in general, you're not really expected to wear anything fancier to an academic conference than you would for a normal day at work or in class.

In practice, most people do tend to dress up a little bit, just to look their best, but still, even if you just wear your normal clean everyday clothes, you're not going to embarrass yourself (any more than you usually do, at least). As the other answers note, "smart casual" is the usual style here.

Nothing says you can't wear a suit if you want to; I see people do that all the time in conferences. Then again, some of those people also wear a suit to work every day, so...

Still, if you're in doubt, why not ask your advisor (or someone else in your group who's been to similar events before)? That's part of their job: to teach you the basics of academic work, which certainly includes presenting your work at a conference.