In Rummy, the Joker is a valuable card. There are two types of Jokers - printed Joker and cut Joker. In addition to the standard 52 playing cards in a deck, there is an extra card with a printed Joker on it.
After dealing cards to all players, the remaining cards are placed face down, and a random card is picked and turned over to serve as the cut Joker. All cards of the same rank from all four suits also become cut Jokers. Both printed Jokers and cut Jokers can be used to replace any missing card when forming valid sequences and sets (excluding pure sequences, which must be formed without a Joker).