Online Bookies or Bookmakers, which are sometimes called Sports Books, will take your money for a bet and you can either win more money or lose your bet.

There are some betting exchanges such as Betfair and BetDaq where you can "lay" bets, i.e. place a bet that something will not happen, i.e. you are like the bookmaker.  You can also bet normally on "to win" bets on these websites and often get better odds than going to an online bookie as you are betting against other "punters".

A good place to check and get the best odds before you place a bet is www.oddschecker.com

Good luck!  Remember not to gamble more than you can afford to lose :)