Poker is a card game in which players wager money on the outcome of a hand. Each player receives five cards, and the best hand wins. The game can be played with a standard deck of 52 cards or with different combinations of cards, depending on the variant being played.

In the earliest forms of poker, a hand consisted of a single card, called the hole card. Then, the player who received the card bet a fixed amount; his or her opponents were then obliged to call it. Over time, players began to play with more than one card in their hands, and the game evolved into a series of betting intervals. At the end of the last betting interval, all remaining bets are collected into a central pot, and the players reveal their cards to evaluate their hands.

At the highest levels of competition, it is not uncommon for a single player to win multiple hands in a row. This form of poker is known as a heads-up hand, and it is usually won by a player with an excellent understanding of the game’s rules and structures.

In the early 20th century, John von Neumann introduced a mathematical model of poker to help solve the problem of optimal betting strategy. He demonstrated that players should bet large with their strongest hands, and with a definable percentage of their weakest hands, in order to earn the highest long-run expected return.