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.
Nothing in this world is free and you will pay for this service, but at a sigificantly reduced rate compared to a bookmaker. The more popular exchange betting sites only charge the winners 5% of their winnings compared to rates like 10% to 20% for bookmakers. This money covers the costs of running the exchange plus a small margin for the owners.