Betfair is the trading name of Betfair Limited, which was founded in August 1999. As you will know, betfair is a betting exchange, which allows you to choose whether to back a selection or instead effectively act as the bookmaker and lay a selection instead. Betfair.com is the World's biggest betting exchange.
If you bet with Betfair at the Cheltenham Festival you can trade on a huge number of markets. Ante-post betting on Cheltenham markets can be found by going to betfair and clicking on Horse Racing > UK > Antepost.