Traditionally betting has occurred between a customer and a bookmaker where the customer ‘backs’ (bets that an outcome will occur) and the bookmaker ‘lays’ (bets that the outcome will not occur). Betting exchanges offer the opportunity for anyone to both back and lay.
For example, if someone thinks Team A will win a competition, he may wish to back that selection. A bookmaker offering the punter that bet would be laying that selection. The two parties will agree the backer’s stake and the odds. If the team loses, the layer/bookmaker keeps the backer’s stake. If the team wins, the layer will pay the backer the winnings based on the odds agreed.
As every bet transacted requires a backer and a layer, and the betting exchange is not a party to the bets transacted on it, any betting exchange requires both backers and layers.