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http://en.wikipedia.org/wiki/Sportsbook

In the United States a sportsbook (sometimes abbreviated as book) or a race and sports book is a place where a gambler can wager on various sports competitions, including golf, football, basketball, baseball, hockey, soccer, horse racing, boxing, and mixed martial arts. The method of betting varies with the sport and the type of game. There are roughly 150 licensed sportsbooks in the United States, all located in Nevada casinos, as such operations are illegal in the other 49 states[citation needed]. Sports betting is fully legal in many countries: for general coverage of the topic see sports betting. In English-speaking countries other than the United States terms such as betting shop or bookmakers are used rather than sportsbook. This article is restricted to sportsbooks in the United States and the impact of internet sports betting on the U.S. market.
Winning bets are paid when the event finishes, or if not finished, when played long enough to become official; otherwise all bets are returned. This policy can cause some confusion since there can be a difference between what the sportsbook considers official and what the sports league consider official. Customers should carefully read the sportsbook rules before placing their bets.
The betting volume at sportsbooks varies throughout the year. Bettors have more interest in certain types of sports and increase the money wagered when those sports are in season. Some major sporting events that don't follow a specific schedule, like boxing, can create peaks of activity for the sportsbooks.