Poker with jokers is somewhat different from regular poker thanks to a few Jokers thrown in for good measure. There can be from 53 to 56 cards in total in Joker games: 52 regular cards + 1 to 4 extra Jokers.
The Joker is treated as a card that helps make any given hand the strongest possible. If a Joker can be used to replace more than one card in order for a player to produce more than one full hand, it is (unsurprisingly!) used to produce the strongest of those possible hands.
When at least one Joker is used to play Texas Hold 'Em, it becomes possible to play a hand called "Five of a Kind", which is made up of five cards of identical nominal rank (such as, say, 5 Tens, with 4 regular Tens and a Joker acting as the "fifth" Ten).
A hand played employing a Joker is considered equal in strength to an identical hand made up entirely of regular cards and without any Jokers.