Card counting is a skill used by blackjack players as a part of their blackjack strategy, more specifically their betting strategy. The purpose of card counting is to give the player an idea of how the cards are flowing in order to manage their betting.
Card counting allows a blackjack player to keep track of whether more low cards have passed out of play, leaving the remainder of the deck rich in high cards and vice versa. Blackjack players know that low cards favor the dealer while high cards favor them.
Knowing which type has mostly been played and what still remains is what helps a blackjack player determine when to raise and lower his wager. For example, if the remaining cards are rich in high cards, the player knows to raise his bet. When the count changes and reveals that the high cards have been played and the remaining deck is rich in low cards, he knows to lower the amount of his wager.
The most basic card counting system, Hi-Lo, assigns low cards a +1 value while high cards have an assigned value of -1.
During the course a game if a lot of low cards have been played the count will become positive. This is because for every low card played the player adds a +1. The more low cards that are played, the higher the count goes in the positive range. A positive count of +3 or higher is a good indicator that the remaining deck is rich in high cards. At this point blackjack players that are card counters will begin raising their bets.
Conversely, when more high cards have been played, adding in -1 for every high card played will bring the count lower and lower until it slips into a negative count. This means that more high cards have been played and that the remaining deck is rich in low cards. This is when blackjack players will lower their bets.
You can see how blackjack players who have practiced this skill can use it to control their betting. This is why card counting factors into the betting strategy portion of a blackjack strategy.
Card counting also influences your blackjack odds. The more money you win from the house, the more you are taking off the house edge because it puts them at a disadvantage and causes them to lose more money.