If you're counting cards in blackjack, that's not cheating, it's just playing to win, and the casino doesn't like it when people do that, so they'll whine about it being cheating and kick you out if they catch you. If you're counting cards in poker, and trying to find which player is the sucker and what their tell is when they've got a good or bad hand, that's not cheating, it's part of the game, because it's a game of skill, not just chance. And faking your own tell may be card sharping, but it's not cheating, nor is having a pretty girl accomplice flirting with the sucker as long as she's not telling you what cards he has.
But if you're marking the cards? Yeah, that's cheating. So is keeping an ace up your sleeve, or using a mirror to look at your opponents' cards, or dealing from the bottom of the deck if you're the dealer, or putting a magnet behind the roulette wheel.
And loading your opponent's six-shooter with blanks for the gunfight that'll happen if he catches you cheatin'? That's way cheating.