Card counting is a technique used by card players, called card counters, typically used in Blackjack games. Players using this technique keep a tally of the cards drawn in order to give them an advantage over the house by keeping track of the low and high cards on the table. By doing this, the card counter believes they can predict the cards yet to be played (therefore, making more money by "outsmarting", and taking away the advantage of, the house.
In one form of card counting, the card counter assigns a number to the cards which have already been played (-1 assigned to 10s, Jacks, Queens, Kings, and Aces; 0 assigned to cards 7,8 and 9; and +1 assigned to cards 2,3,4,5 and 6). The higher the count (meaning more +1 cards have been played) the more likely a high card (10 through Ace) will show.
An example of card counting is as follows:
Player A has a 6 (+1)
Player B has a 2 (+1)
Player C has a 5 (+1)
Dealer has a 8 (0)
The chances of the next card being high is great given the tally is +3.