Although counting cards is considered to be cheating, it actually isn't. After spending so much time learning and practicing card counting, it is really just the matter of applying the strategy. You are not harming anyone, except, maybe, casino odds. Nevertheless, it is still the matter of applying your own skills, and information gathered using your own abilities.
Anyway, casinos aren't very happy about card counting, and the reason for that is quite obvious, isn't it? If they suspect you are counting cards, they could escort you out of the casino.
Some other things the players attempt to do while playing blackjack are in different category. Here we are talking about the real cheating.
Marking the cards is quite popular - they put their nail prints on different parts of cards I order of recognizing them in the future. That's how they can guess the dealer's faced down card.
Another popular activity is switching the cards. It is much harder thing to do, with all the security measures present in today's casinos, including guards, cameras etc, but really skilful players are capable of performing such actions.
The only thing is that, if you are caught, you will be not only escorted out of this casino, but probably from all of them, and you can face some criminal charges, in addition.
The dealers can be bribed, not only that the players can pay them to work at their advantage; the casinos can also try the same thing. Although, the chances are on their side, anyway, and they have a great deal to lose.
In case the dealer has a deal with a player, and they got caught, he will lose his job for sure, and maybe no other casino will ever employ him. The player will be thrown out, probably for good.
If the players aren't counting cards, casinos are going to keep their advantage, when we are talking about Blackjack. There are no other successful ways for cheating in that.
In the end, all of this just cannot be done in online casinos; you cannot touch the cards and you cannot close the deal with the dealer.