Original: In a casino, a house dealer handles the cards for each hand, but the button (typically a white plastic disk) is rotated clockwise among the players to indicate a nominal dealer to determine the order of betting.
Simplified: In a casino a "house" dealer handles the cards for each hand, but a button (any small item used as a marker, also called a buck) is rotated among the players to determine the order of dealing and betting in certain games. In a home game, the right to deal the cards typically rotates among the players clockwise, but a button may still be used.