Card counting is widely talked about because done correctly it gives the player an advantage over the house. The result is that a lot of people jump in and decide that they are going to be card counters. The reality however is that it is much more difficult to count cards than most people realize and there are very few people who can do it correctly. If you are going to do it you will need to put in a great deal of practice.
The theory behind card counting is that it has been well established that the odds are in favor of the player when there are a lot of high cards, particularly cards with a value of ten still in the deck. The odds are worst for a player when the deck contains a large number of low cards. Therefore if you can determine when the deck has a lot of high cards you can increase the amount that you bet when the odds are in your favor. Done correctly this can actually give you an edge over the casino.
The first thing that you need to be aware of if you are counting cards is that it is very difficult. There are a number of systems that are used to track cards but they all require that you have a way of keeping track of every card that gets dealt between the times when the deck is shuffled. This sounds easy in theory but is very hard to do in practice. The game moves very fast and you not only have to keep track of every card that gets dealt to every player you also have to play your hand with proper strategy. It takes a lot of practice to be able to count cards and few people can do it well.
Strictly speaking card counting is not illegal however you will find that most casinos strongly discourage it. If they suspect that you are counting cards you will normally be asked to leave. This makes the whole process even more difficult since you will need to be able to count cards without anybody noticing that you are doing it. You will also have to keep the amount that you vary your bets limited which lowers the benefits that you get from counting cards.
The other issue that you need to keep in mind if you are going to count card is that the casinos have started using multiple decks for blackjack. This not only makes counting cards more difficult but it also reduces the benefits even if you do get it right. Using more decks means that the proportion of high cards to low cards in the deck will remain closer to even than it would if a single deck is used. This makes the value of counting cards much lower than it was in the past when only a single deck was used.