Put simply, matched betting is betting to avoid a loss (or at least make a small loss). The idea is to match any bet you put on at the bookies with an equivalent lay bet on a betting exchange (such as Betfair). If you win at the bookies, you lose at Betfair (and vice-versa) and the two bets cancel each other out. So, why would you want to do that? Well, the bookies will pay you a bonus for signing up (or re-depositing) so if you can bet with them with no risk, then you can easily claim the bonus. It's a pretty easy way to make some extra money, and even The Guardian thinks so.
A quick list of some boomakers to get started with. We have a full list, along with an explanation of each one's terms and conditions, inside the members' area.