If you've ever heard of YouGov polls, and wondered who these people are, then follow the link. YouGov are a polling organisation which maintains a panel of internet users. From time to time, they email you a link to a poll, typically they pay 50p for each poll completed - though this can vary. Sometimes they pay more, sometimes it's a 'prize poll' - I don't like those myself (i.e. I've not won).
Whether you're emailed a link or not depends upon your profile, they try to get responses that reflect the demographic of the public.
Each poll is opt-in, there's no obligation. I've been doing YouGov polls myself for a while now, and they're pretty quick to do. There have been a few times where I've aborted as I was unhappy about the questions, but for the most part it's fairly innocuous.
Why am I making this post? Well, if you sign up using this link, I get some credit when you complete surveys (at no cost to you).Well, I'm not going to give them a free plug out of the goodness of my heart, am I? At the time of writing, you get £1 for signing up in the first place, plus any regular payments for completing surveys.