Unlike traditional bookies' Starting Price, there is no margin for profit built into the Betfair Starting Price so it often gives you better odds. Commission is the amount you are charged by Betfair on your net winnings on an Exchange market.
Instead of the bookmaker setting the odds, users set the odds and bet against each other, with Betfair taking a percentage commission as their profit for providing the service. Usually, Betfair Exchange is where you will be 'laying' your bets, and the bookmaker is where you will make your 'backing' bets.
