Jack: The lowest of the face cards.

Jackpot: Some online rooms offer a jackpot when a player makes a certain hand or loses with a strong hand.  The jackpot amount can rise throughout the day or be a predetermined amount.

Jacks or Bettor: A poker game where a player must have at least a pair of jacks to open the betting.

Jam: To bet or raise the maximum allowed, especially in no-limit poker.

Joker: The joker is used in some games of poker.  It is a wild card that can represent any card the player using it wants to.

Juice: A percent of the pot kept by the card room.  It is also called the rake, and can refer to someone marking the deck.