Unlike traditional bookmakers, who employ people to compile the odds that they offer, Betfair doesn’t set the odds. Instead it is Betfair customers who set the odds. This provides three main benefits:
If you want to back a selection, the odds are likely to be better than a traditional bookmaker, because punters are setting prices on Betfair markets based on their opinions and (unlike a traditional bookmaker) they do not build a margin into their prices.
On Betair you can back your selection to win in the “traditional” manner. Alternatively you can take the other side of the market and back a selection not to win (or “lay”).. The ability to back or lay a selection provides the added benefit of allowing you to “trade” a position and lay your bet back if it looks the likely winner, to guarantee yourself a profit.

With Betfair you can back or lay right up to the final whistle of a football match or up to the last second of a horse race, at odds set by other punters.