Most bookmakers in the USA bet on college and professional sports, though in the UK they offer a wider range of bets, notably on political elections.
By adjusting the odds in his favour or by having a spread, the bookmaker will aim to guarantee a profit by achieving a 'balanced book', either by getting an equal number of bets for each outcome, or (when he is offering varing odds) by getting the amounts wagered on each outcome to reflect the odds. When a large bet comes in, a bookmaker can also try to lay off the risk by buying bets from other bookmakers.
One of the most infamous bookmakers was Robert Angleton of Houston, Texas. Not only was he a bookie, but he also was a police informant about his smaller rivals. When they were shuttered, he took their business. His bookmaking scheme ended with the death of his wife, Doris Angleton.