Back bets are stakes placed on an event occurring. For example in Exchange poker, you might Back a hand to win. That means if the hand wins, so do you. You can also ask for better odds, or more money than currently available. In either case your bet will appear on the Lay side of the market.
Lay bets are the opposite to backing, so you are putting money on the event not occurring or in the example above, you are backing the hand to lose. The Lay side of the market shows the money and odds requested by backers. To accommodate a backer, click a button in the Lay column.