Card counting is a card game strategy used to determine when a player has a probability advantage. The term is used almost exclusively to refer to the tracking of the ratio of high cards to low cards in blackjack.
A card-counting iPhone application designed to help players win at blackjack has just come out and is already being banned from Las Vegas casinos.
Card counting in general is not illegal in Nevada, but using a device to help you count cards is considered a felony.