blackjack 21 movie

How does the technique called "card counting" at work?
I recently saw a trailer for the upcoming movie called 21. In the film, some children learn something called "counting cards to win money at blackjack. If someone can please tell me the trick.
Counting cards is not really a trick. This is not cheating and not illegal. However, the casinos in general, prevents him from playing and ask you to leave or comp to cut their losses. In essence, a player keeps track of the cards have been distributed, with attention to the number of cards, aces and 9 and 10 he played. Statistically, the face more cards in play, the more likely the player and not the house. Here is a page that can explain better than me. http://en.wikipedia.org/wiki/Card_counting
21 movie, Mike Aponte Leader of MIT Blackjack Team