Counting cards in blackjack is a way to increase your chances of winning. If you're good at it, you can actually take the odds and put them in your favor. This works because card counters raise their bets when a deck rich in cards that are advantageous to the player comes around. As a general rule of thumb, a deck rich in 10's is better for the player, because the dealer will bust more often, and the player will hit a blackjack more often. Most card counters keep track of the ratio of high cards, or 10's, by counting them as a +1 or a -1, and then gives the opposite +1 or -1 to the low cards in the deck. Some systems use a balanced count where the number of low cards is the same as the number of 10's.

But the most interesting card to me, mathematically, is the 5. There were card counting systems back in the day that involved doing nothing more than counting the number of fives that had left the deck, and when the 5's were gone, the player had a big advantage and would raise his bets. A good basic strategy player is getting a 99.5% payback percentage from the casino. Every 5 that's come out of the deck adds 0.67% to the player's expected return. (In a single deck game, anyway.) That means that, all other things being equal, having one 5 gone from the deck gives a player a small advantage over the house.