A suit is an additional property of a card alongside its rank. There are four suits in a standard deck: clubs, diamonds, hearts, and spades.

Use

Unlike the rank, the suit plays no role in the ordering of cards or in the comparison of hands of the same type. Other than being the criteria by which a flush is formed, the suit plays no role whatsoever.

In particular, contrary to the popularity of the idea, the A♠ is not worth any more than any of the other aces A♣ A♦ A♥.

In 7 card stud there is a very particular exception that should two players have the same high card the suit is used to determine who bets first. This exception is only for starting that betting and has no influence on the hand values.

Colour

Clubs and spades are black cards. Diamonds and hearts are red cards. The reason behind only having two colours is uncertain but may have to do with limited printing abilities. This limited distinction has often led players to miss a flush, either tossing a winning hand or taking an expensive beat to their straight.

Most online card rooms will also allow you to play with a four coloured deck. In such a deck the diamonds are blue and the clubs are green. Attempts to introduce such decks in live casinos has never been successful.