Use of Jokers in Poker
I don't think you'll find jokers used in live poker (against other humans) in a casino. However, there are many variations of poker played in home games that use jokers and/or other wild cards. In home games, the rules are quite varied because usually the person suggesting the game (dealer's choice) gets to make whatever rules they'd like. Typically though, the Joker is treated as a wild card which you can assign any value you wish. Scoring (hand ranking) is done identically to without wild cards, except that now 5 of a kind is possible, and that is the best hand. (5 deuces beats a royal flush, and 5 Aces is the best possible hand.)

This page lists some home poker games that use jokers.