In a standard deck, there are usually two Jokers.
The Joker's use varies greatly. Many card games omit the card entirely; as a result, Jokers are often used as informal replacements for lost or damaged cards in a deck by simply noting the lost card's rank and suit on the joker.
Other games, such as a 25-card variant of Euchre which uses the joker as the highest trump, make it one of the most important in the game.
Often, the joker is a wild card, and thereby allowed to represent other existing cards. The term "joker's wild" originates from this practice, as does the game show of the same name. The Joker Is Wild is also the name of a 1957 film starring Frank Sinatra.