Illegal gambling in the U.S. is a federal offense.  Illegal gambling is identified as gambling practices that take place in an unlicensed establishment and are done for the profit of an individual.  By this definition, friendly neighborhood pick-up poker games are not illegal, as they are not performed as a business.
Illegal gambling can take the shape of dog fighting or racing, placing bets on game outcomes, or established poker games for profit.  While it is considered a federal crime if conducted as a business, each state has rules and regulations defining illegal gambling and what the punishment is for breaking the law.  A person may also be found in violation of illegal gambling regulations if they are under the legal gambling age set by the state of residence.
Due to the unregulated practices of online gambling, there is a movement to make online gambling illegal.  There have also been cries that certain network television game shows partake in illegal gambling.  Such shows that request viewers to send a .99 cent text message with the answer to a question in order to possibly win something have been brought under review.  Some raffles can even be considered illegal gambling and therefore whoever is hosting the raffle must be aware of what the state legislation dictates.
Many people feel that opening more legal casinos and gambling establishments will cut back on illegal gambling.  This has not been the case though, as recent studies have found.  People interested in illegal gambling are either interested in betting on sports or activities that are not allowed by any state (such as dog fighting) or they feel that they can stake less money and win more money by gambling in illegal operations.