What is backing and laying?
On Betfair you can back and lay. To 'back' a selection means you think it will happen, like you'd do at a traditional bookie. Or you can take the opposing viewpoint and 'lay' if you think your selection will not happen.
Every fixed-odds bet, whether it's with your local High St bookie, a betting exchange or even two mates in a pub, requires a backer and a layer, you've probably just never thought of it that way.
Imagine two guys betting a pint on the Merseyside derby - one cheering Everton, one cheering Liverpool. The Everton supporter is automatically opposing, or laying, Liverpool if he decides to go with his heart. Likewise the Liverpool fan backs his team for a pint, but is laying Everton in the same process.
Lay betting is the other half of the deal from a back bet. Both are bets but opposing views.