Betting exchanges are emerging as the best sportsbetting sites because they offer better odds than traditional bookmakers and you don't see as much of your betting profit disappearing into the bookie's pockets. Call it reduced juice sportsbetting if you wish.
In basic terms, you can either place a bet just like you would with an online sportsbetting site, or you can be the bookie and post your odds and stake and people can choose to match it. The former is known as a back (or accept) and the latter is a lay (or offer). Think of it in the same terms as a stock exchange which is a forum for buyers and sellers of stocks. The bet exchange sites keep track of the lays and backs in the order they are matched.
Below are a few betting exchanges and you can see many more at Set Your Odds. If you are new to the concept of a betting exchange and wish to only pay between 1% and 5% commission on your winnings then you will find a lot of information at this site. Click to see betting exchange reviews.