The Jack matches any other card of any suit. Additionally, when you play a Jack, you are expected to select a suit which the next player must follow.
Being a wild card, it comes with one exception: it can only not be played onto another Jack.
Another specialty of this very special card is that, if you end the game with one or more Jacks as your last card(s), this is called a "Mau Mau" victory. See more details in chapter The Last Card.
Be warned, each Jack is counted 20 in your hands when you lose a game, which is double the size of any other card. It is therefore risky to keep a Jack in your hand for too long when the end of the game is near.