Entering a poker tournament requires a single buy-in, once you’ve registered you’ll be given chips to play with and if you lose them all you’ll be eliminated - sometimes, though, you can purchase a rebuy for the same cost as your initial buy-in. Cash games, however, are different in the respect that after buying in for at least the table minimum, each bet you place will be with actual cash. You can play for as long as you want or leave at any point.