The game of blackjack differs from most other casino games since it is a "beatable" game. An average player who invests some time and masters some card-counting techniques can diminish the house edge in blackjack. Card counting allows the player to track the ratio of high value cards to low value cards left in the deck. The reason for doing this is because high value cards benefit the player while low value cards benefit the dealer. Counting the cards during a blackjack game can give you more accurate information about which cards are still left in the deck and you can find out the probability of being dealt a card that benefits your hand.
In 1962, a professor of mathematics named Edward O. Thorpe revolutionized the approach to blackjack when he published his book "Beat the Dealer". Thorpe used the best data in the world, then located at MIT University, to test his theory by running automated tests that simulated blackjack games. He incorporated his card-counting techniques into tests and showed that it worked. With his book, it was for the first time proved mathematically that a player can overcome the small advantage the house has in blackjack by using card counting.
A player does not have to be a mathematical genius to be able to count cards. This is why so many players have used card counting when playing blackjack. Card counting is not an illegal activity, but casinos dislike it and take the right to refuse card counters to play. The reason for this is that when card counting is used the house loses its advantage and the game times the player. Casinos are also notorious for taking players suspected of card-counting into the casino's back room to check their IDs and informing them that they are no longer welcome at the blackjack tables. In the old days, these back rooms were a place where casino thugs really taught these card counters a lesson.