Card counting is something you can theoretically do to beat the house edge. The BBC did documentary about it.
BBC – Horizon – 2004 – Blackjack Card Counting by DocumentaryHD2014
Basically, the odds of the game are not static. Its not like roulette where every spin is independent. With blackjack, the cards that have gone before affect the game. When the deck is full of high cards, 10s and aces, the player has an advantage. Its this what players exploit.
The key is to bet low or not play when the deck is low on tens and bet max when the deck is high. There are different ways of card counting but they all work on the same principle. Casinos can spot this betting pattern quite easily though so ban players for doing it.
Casinos have got wise to this and do things to stop you counting. Firstly, shuffling after every deal makes counting impossible. In RNG blackjack, the deck is shuffled after every hand. Using more decks reduces the scope for card counting too. In some land casinos, they have continuous shuffling machines which stop you being able to count cards.
It is possible to count cards at the live blackjack tables online. The casinos do protect themselves by using many decks and re-dealing well before the end of the deck. The betting patterns would also be a bit odd too. As online casinos take note of all bets and hands, they would quickly ban you if you made enough money.