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 bets 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. The action then proceeds to the left. Each player in turn must either match the maximum previous bet or fold, losing all further interest in the hand. A player who matches a bet may also raise, increasing 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, the remaining player collects the pot without showing his hand. If more than one player remains in contention after the final betting round, the hands are shown and the winning hand takes the pot.David G. Schwartz, Roll the Bones: The History of Gambling, Gotham (2007), ISBN: 978-1592403165

Players feel that this betting system distinguishes poker from gambling games. With the minor 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 is determined mostly by chance, the long-run expectations of the players are determined mostly by their actions chosen based on probability and psychology.

Variations

WSOP Main Event Table Poker has many variations, all following a similar pattern of play and generally using the same hand ranking hierarchy. There are three main families of variants, largely grouped by the protocol of card-dealing and betting:

• "Straight": A complete hand is dealt to each player, and players bet in one round, with raising and re-raising allowed. This is the oldest poker family; the root of the game as currently played was a game known as Primero, which evolved into the game Three-card brag, a very popular gentleman's game around the time of the American Revolutionary War and still enjoyed in the U.K. today. "Straight" hands of five cards are sometimes used as a final showdown, but poker is currently virtually always played in a more complex form to allow for additional strategy.
• Stud: Cards are dealt in a prearranged combination of face-down and face-up rounds or "streets", with a round of betting following each. This is the next-oldest family; as poker progressed from three to five-card hands, they were often dealt one card at a time, either face-down or face-up, with a betting round between each. The most popular stud variant today, Seven-card stud, deals two extra cards to each player (three facedown, four faceup) from which they must make the best possible 5-card hand.
• Draw: A complete hand is dealt to each player, face-down, and after betting, players are allowed to attempt to change their hand (with the object of improving it) by discarding unwanted cards and being dealt new ones. Five-card draw is the most famous variation in this family.
• Community: A variation of Stud, players are dealt an incomplete hand of face-down cards, and then a number of face-up "community" cards are dealt to the center of the table, each of which can be used by one or more of the players to make a 5-card hand. Texas hold-em and Omaha are two well-known variants of the Community family.