Counting cards is not as hard as it sounds, though you will have to be able to do the math very quickly. The basic idea is that you are watching what cards are on the table so that you can know if the odds favor you getting the card that you want or not based on what is left in the deck. For instance, pretend that you are playing with one other player. He has a six, a Jack, and a Queen, and he has busted out already. The dealer has a Queen up, and you do not yet know what the other card is. You are holding a third Queen, and a five, and you are trying to decide whether you want to hit or stand.
The first thing to think about is that anything over a six will lose you the game. In a deck of fifty-two cards, that means that you would have twenty-eight cards that could cause you to lose; this takes into account the seven through the King in every suit. You would also have twenty-four cards that would cause you to not bust — you would not necessarily win, but you would go up without going over. This takes into account everything from the six down to the Ace in every suit. Counting the cards, then, is the process of determining what really is left in the deck based on what is left on the table. In the above example, three queens and a jack are visible. This reduces the amount of cards that will make you lose to twenty-four. The dealer’s other card is unknown, so you have to ignore it. However, the six and the five are also visible. This reduces the amount of cards that will allow you not to bust to twenty-two. Therefore, there are twenty-four cards left that will make you lose verses twenty-two cards that will allow you not to lose. You are smarter, if playing the odds, to hold onto what you have.
This process can be expanded for all of the hands on the table; if many people are playing at once, you will have to see how many cards are out and add up what numbers they are. If you can see all of the twos, threes, and fours — unlikely, but it should be thrown out there for the sake of the example — you can reduce the amount of cards that will allow you to not bust by twelve. You also have to take into account the odds that the dealer has of going over your card total. If the dealer has a queen already up, anything between a six and on up to an Ace will cause you to lose. Therefore, you may want to gamble and Hit anyway, since your fifteen will likely not stand up. This is the big risk and skill of Blackjack.