Sports betting is the general activity of predicting sports results by making a wager on the outcome of a sporting event. Perhaps more so than other forms of gambling, the legality and general acceptance of sports betting varies from nation to nation. In the United States, the Professional and Amateur Sports Protection Act of 1994 makes it illegal to operate a "betting, gambling or wagering scheme", except for in the states of Delaware, Nevada, Montana and Oregon. Nevada, however, is the only state currently allowing sports gambling, while in many European nations bookmaking (the profession of accepting sports wagers) is highly regulated but not criminalized.

Odds
Odds for different outcomes in a single bet are presented either in European format (decimal odds), UK format (fractional odds), or American format (moneyline odds). European format (decimal odds) are favoured in continental Europe, Canada, and Australia. They are the ratio of the full payout to the stake, in a decimal format. Decimal odds of 2.00 are an even bet. UK format (fractional odds) are favoured by British bookmakers. They are the ratio of the amount won to the stake. Fractional odds of 1/1 are an even bet. Moneyline odds are favoured in the United States. They are the amount won on a 100 stake when positive and the stake needed to win 100 when negative. Moneyline odds of 100 are an even bet.

Unlike point spread bets, a moneyline wager requires only that the team wagered upon win the match. In sports such as baseball, where certain teams can be heavy favorites against weaker opponents (sometimes as much as -350 or higher), the moneyline system requires that a hefty sum be risked on the favorite, while enticing underdog players with a higher payout.

Many bookmakers offer several alternative bets, including the following:

• *Proposition bets. These are wagers made on a very specific outcome of a match. Examples include guessing the number of goals each team scores in a soccer match, betting whether a wide receiver in a football game will net more or less than a set amount of total yardage, or wagering that a baseball player on one team will accumulate more hits than another player on the opposing team.
  * Parlays. A parlay involves multiple bets (usually up to 12) and rewards successful bettors with a large payout. For example, a bettor could include four different wagers in a four-team parlay, whereby he is wagering that all four bets will win. If any of the four bets fails to cover, the bettor loses the parlay, but if all four bets win, the bettor receives a substantially higher payout (usually 10-1 in the case of a four-teamer) than if he made the four wagers separately.
  * Teasers. A teaser allows the bettor to combine his bets on two or more different games. The bettor can adjust the point spreads for the two games, but realizes a lower return on the bets in the event of a win.
  * Run line, puck line or goal line bets. These are wagers offered as alternatives to straight-up/moneyline prices in baseball, hockey or soccer, respectively. These bets feature a fixed point spread that offers a higher payout for the favorite and a lower one for the underdog. For example, the above-described Cardinals/Cubs baseball game might offer a run line of St. Louis -1.5 (+100) and Chicago +1.5 (-120). A bettor taking St. Louis on the run line can avoid risking $200 to win $100 on the moneyline, but will collect only if the Cardinals win by 2 runs or more. Similarly, a run line wager on the Cubs will pay if Chicago loses by no more than a run, but it requires the bettor to risk $120 to win $100.
  * Future wagers. While all sports wagers are by definition on future events, bets listed as "futures" generally have a long-term horizon measured in weeks or months; for example, a bet that a certain NFL team will win the Super Bowl for the upcoming season. Such a bet must be made before the season starts in September, and winning bets will not pay off until the conclusion of the Super Bowl in January or February (although many of the losing bets will be clear well before then and can be closed out by the book). Odds for such a bet generally are expressed in a ratio of units paid to unit wagered. The team wagered upon might be 50-1 to win the Super Bowl, which means that the bet will pay 50 times the amount wagered if the team does so.
  * Head-to-Head. In these bets, bettor predicts competitors results against each other and not on the overall result of the event. One example are Formula 1 races, where you bet on two or three drivers and their placement among the others. Sometimes you can also bet a “tie”, in which one or both drivers either have the same time, drop out, or get disqualified.
  * Totalizators. In totalizators (sometimes called flexible-rate bets) the odds are changing in real-time according to the share of total exchange each of the possible outcomes have received taking into account the return rate of the bookmaker offering the bet. For example: If the bookmakers return percentage is 90%, 90% of the amount placed on the winning result will be given back to bettors and 10% goes to the bookmaker. Naturally the more money bet on a certain result, the smaller the odds on that outcome become.

The Internet not only revolutionized the ability to bet online, but also the ability to communicate with like-minded bettors. Sports betting forums offer lively give and take where bettors discuss their predictions about games and help one another decide on profitable bets. Additionally a cottage industry of tipster sites has sprung up where inexperienced bettors can, for free or a fee, receive previews and tips for upcoming events. Free sites are generally affiliated with online bookmakers and finance their service by introducing new customers to their preferred online bookmakers.