Joker
A joker is a card that works as a wild card in some games. It can stand for any card its holder wants, or only for certain cards, depending on the rules.
The simple explanation
Standard poker is played without jokers. Some draw games and video poker use one joker as a 53rd card.
A closer look
In many high draw games, the joker may be used as an ace, or as a card that completes a straight or flush, and in low games as the lowest card that is not already in the hand. This is called the bug in some rules. Always ask what the joker can be before you play, since rules differ.
A practical example
A home game uses one joker in five-card draw, and a player uses it as an ace to make a pair.
Full explanation
A joker is a card that works as a wild card in some games. It can stand for any card its holder wants, or only for certain cards, depending on the rules.
A joker is a special card that can be wild.
Standard poker does not use it.
Rules differ for what it can represent.
Ask before you play.