Card counting is a well known system first made famous by mathematician Dr Edward Thorp who is considered the father of card counting, after publishing a book, Beat the Dealer in 1962. The system involves recording the number of high value cards that have been exposed during the course of the hand.
If you know that there is a large amount of high value cards remaining in the pack, it enables you to place large wagers with the expectation of being dealt a high card.
This method proved to be hugely profitable if played correctly. However, it is extremely difficult to perfect and has been banned by all casinos and havening developed methods of detecting card counting, once suspected, you will simply be escorted out of the casino.
Most casinos have developed countermeasures by, shuffling prematurely and frequently and have started introducing automated card shufflers which shuffle after every hand. It’s unfortunately completely useless online, since the deck is virtually shuffled after every hand.