I've had visitors on this site searching for information on "how to count cards in poker" so I thought I would write about the subject.
First of all, there's no way to count cards at poker in a way similar to blackjack since the deck is shuffled after every hand and what happened in the hand before has nothing to do with what's going to happen in the next hand.
Card counting at poker must therefore mean the same as "how to count outs," or in other words, "how to count cards that you can win with." Counting outs in poker consists of calculating all the unseen hands, the hands with winning potential and then comparing those groups between each other.
So how does counting cards at poker work? Say you're on the turn with a flush draw and you wonder how likely it is for you to hit a flush on the river. There are 52 cards in a standard card deck and 13 cards of each suit. Since you're on a flush draw, you have four of the same suit meaning you'll need to hit one of the remaining nine cards to get your flush. And since you're on the turn, you've already seen four cards (on the board) plus two cards (in your hand) meaning there are 46 cards to be seen.
A simple calculation shows us that 9/46 = 0.20 so you have a 20% chance to hit a flush on the river. (This way of calculating works for the most popular poker game, Texas Hold'em.)
Not "double-counting" outs is important to remember when learning how to count cards in poker.
For example, say you have 6-5 of hearts and there are 2-7-8-J on the board with two of them being hearts. Now you have a flush draw and a straight draw. How many outs do you have? 46 cards unseen, nine flush outs and all the fours and nines so eight straight outs combined (since there are four cards of each value). Nine outs plus eight outs means you'd have 17 outs altogether. However, two of your straight outs are flush outs at the same time (4 of hearts and 9 of hearts) which means subtract two of your total outs in order to not to double-count leaving you with 15 outs in total.