Thanks to Hollywood, everyone has an idea about counting cards at the casino. It's a technique in blackjack that can bring the odds into your favor by a tiny percentage. But enough to make a difference over the long term. The payer needs to keep a real and a running count in his head at the same time. From this, he'll know the type of cards left in the deck. Though counting cards is not illegal, and you'll not get arrested, as you can imagine, the casinos are not enamored with card counters.
Compared to many other casino table games, the blackjack house edge appears to be much lower. In fact, in a regular game it hovers at around 2%. Because blackjack is a game of both luck and skill, it's quite possible, that by playing perfect blackjack strategies, you can lower this down to around 0.5%. Only by using card counting can you get to this amazing figure. But the truth of the matter is that the counting of cards is very difficult at a land-based casino, and impossible online.