Matched betting is a virtually risk free method of milking the free bets offered by bookmakers to entice customers to bet with them. It uses a betting exchange and a bookmaker's account to back both possible outcomes for a minimal loss and then be given a free bet by the bookmaker in return for betting with them.
I say "virtually" because you need to be careful and double check all your figures and entries before you finalise them. In addition prices can move, events can start before you get both entries made and if doing it in-play, a event can skew the odds away from you.