card counting is when you use a particular method to gain the upper hand in a card game. The actual term is used mainly because of the game black jack and it became famous when people were able to work out out the ratio of high cards to low cards in the game. Now it is used in other games like bridge and spades but it will be notoriously be known from blackjack. There are many forms of card counting now but the best example and funny example is the film Rain Man with Tom cruise and his dyslexic brother.