A card game is any game using playing cards as the primary device with which the game is played, be they traditional or game-specific.
Countless card games exist, including families of related games. A small number of card games played with traditional decks have formally standardized rules with international tournaments being held, but most are folk games whose rules vary by region, culture, and person.
A card game is played with a deck or pack of playing cards which are identical in size and shape. Each card has two sides, the face and the back. Normally the backs of the cards are indistinguishable. The faces of the cards may all be unique, or there can be duplicates. The composition of a deck is known to each player. In some cases several decks are shuffled together to form a single pack or shoe.
Games using playing cards exploit the fact that cards are individually identifiable from one side only, so that each player knows only the cards they hold and not those held by anyone else.