In poker, players form sets of five playing cards, called hands, according to the rules of the game. Each hand has a rank, which is compared against the ranks of other hands participating in the showdown to decide who wins the pot. In high games, like Texas hold 'em and seven-card stud, the highest-ranking hands win. In low games, like razz, the lowest-ranking hands win. In high-low split games, both the highest-ranking and lowest-ranking hands win, though different rules are used to rank the high and low hands.

Each hand belongs to a category determined by the patterns formed by its cards. A hand in a higher-ranking category always ranks higher than a hand in a lower-ranking category. A hand is ranked within its category using the ranks of its cards. Individual cards are ranked, from highest to lowest: A, K, Q, J, 10, 9, 8, 7, 6, 5, 4, 3 and 2. However, aces have the lowest rank under ace-to-five low or ace-to-six low rules, or under high rules as part of a five-high straight or straight flush. Suits are not ranked, so hands that differ by suit alone are of equal rank.

There are nine categories of hand when using a standard 52-card deck, except under ace-to-five low rules where straights, flushes and straight flushes are not recognized. An additional category, five of a kind, exists when using one or more wild cards. The fewer hands a category contains, the higher its rank. There are ${\displaystyle {\begin{matrix}{\frac {52!}{(52-5)!}}=311{,}875{,}200\end{matrix}}}$ ways to deal five cards from the deck but only ${\displaystyle {\begin{matrix}{\frac {52!}{(52-5)!5!}}=2{,}598{,}960\end{matrix}}}$ distinct hands, because the order in which cards are dealt or arranged in a hand does not matter. Moreover, since hands differing only by suit are of equal rank, there are only 7,462 distinct hand ranks.

A royal flush or royal straight flush is a hand that contains an ace-high straight flush, such as A♦ K♦ Q♦ J♦ 10♦, and is the best possible hand in ace-high games when wild cards are not used. A five-high straight flush, such as 5♥ 4♥ 3♥ 2♥ A♥, is called a steel wheel and is both the best low hand and usually the best high hand of the showdown in ace-to-five high-low split games.

A straight flush is a hand that contains five cards of sequential rank, all of the same suit, such as Q♥ J♥ 10♥ 9♥ 8♥ (a "queen-high straight flush"). It ranks below five of a kind and above four of a kind. Under high rules, an ace can rank either high (as in A♥ K♥ Q♥ J♥ 10♥, a royal flush) or low (as in 5♦ 4♦ 3♦ 2♦ A♦, a five-high straight flush), but cannot simultaneously rank both high and low (so Q♣ K♣ A♣ 2♣ 3♣ is an ace-high flush, but not a straight). Under deuce-to-seven low rules, an ace always ranks high (so 5♠ 4♠ 3♠ 2♠ A♠ is an ace-high flush). Under ace-to-six low rules, an ace always ranks low (so A♥ K♥ Q♥ J♥ 10♥ is a king-high flush). Under ace-to-five low rules, straight flushes are not possible (so 9♣ 8♣ 7♣ 6♣ 5♣ is a nine-high hand).

Each straight flush is ranked by the rank of its highest-ranking card. For example, 10♣ 9♣ 8♣ 7♣ 6♣ ranks higher than 8♥ 7♥ 6♥ 5♥ 4♥, which ranks higher than 6♠ 5♠ 4♠ 3♠ 2♠. Straight flush hands that differ by suit alone, such as 7♦ 6♦ 5♦ 4♦ 3♦ and 7♠ 6♠ 5♠ 4♠ 3♠, are of equal rank.

Four of a kind, also known as quads or four cards, is a hand that contains four cards of one rank and one card of another rank (the kicker), such as 9♣ 9♠ 9♦ 9♥ J♥ ("four of a kind, nines"). It ranks below a straight flush and above a full house.

Each four of a kind is ranked first by the rank of its quadruplet, and then by the rank of its kicker. For example, K♠ K♥ K♣ K♦ 3♥ ranks higher than 7♥ 7♦ 7♠ 7♣ Q♥, which ranks higher than 7♥ 7♦ 7♠ 7♣ 10♠. Four of a kind hands that differ by suit alone, such as 4♣ 4♠ 4♦ 4♥ 9♣ and 4♣ 4♠ 4♦ 4♥ 9♦, are of equal rank.