Many people think that to be a card counter you have to have a photographic memory and remember every single card that's been played. Fortunately, it's not quite that difficult. Actually, the main concept behind card counting is the assumption that the dealer will bust more often when there are a lot of 10's in the deck and that he will complete more hands when there are a lot of smaller cards in the deck.
Now, if you stop to think about it, it makes sense doesn't it? After all, the dealer has to play by set rules that make him take a card until he has a total of 17 or more. If there are a lot of 2's, 3's and 4's in the deck the dealer won't bust very often when he draws cards, but if there are a lot of 10's in the deck then chances are he will bust more often when he is forced to draw cards.