Card Counting is a great way to get thrown out of a casino and not nearly as profitable and easy to learn as tv shows make it seem to be.
That being said let me add that successful card counting is also the only way to beat casinos without going the way of bonus whoring.

There exist many card counting systems and each of them claims profibility, but most are very hard to learn.
It is not possible to explain a complex working card counting system in a short article like this one, but I'll try to show you the basics of most systems.

For every blackjack system it is assumed that high cards left in the deck are good for the player and low cards left in the deck are bad for the player.
Having said that, let's take a look at an example now to make this better understandable.

Ken Uston's Plus/Minus blackjack strategy assigns a value of +1 to 3,4,5,6 and 7 and a value of -1 to tens and aces. Everything else is neutral.
The player then has to change his bets and playing style slightly to get the best value out of playing depending on the count.

Through changing the bet sizes, though, most casinos will recognize a player as a card counter and will throw them out of the room, so be careful when trying to count cards.