Poker is a family of card games that share betting rules and usually (but not always) hand rankings. Poker games differ in how the cards are dealt, how hands may be formed, whether the high or low hand wins the pot in a showdown (in some games, the pot is split between the high and low hands), limits on bet sizes, and how many rounds of betting are allowed.
In most modern poker games, the first round of betting begins with some form of forced bet by one of the players. In standard poker, each player is betting that the hand he has will be the highest ranked. The action then proceeds clockwise around the table and each player in turn must either match the maximum previous bet or fold, losing the amount bet so far and all further interest in the hand. A player who matches a bet may also "raise," or increase the bet. The betting round ends when all players have either matched the last bet or folded. If all but one player fold on any round, then the remaining player collects the pot and may choose to show or conceal their hand. If more than one player remains in contention after the final betting round, the hands are revealed and the player with the winning hand takes the pot. With the exception of initial forced bets, money is only placed into the pot voluntarily by a player who, at least in theory, rationally believes the bet has positive expected value. Thus, while the outcome of any particular hand significantly involves chance, the long-run expectations of the players are determined by their actions chosen on the basis of probability, psychology and game theory.
