The person who deals the cards and runs the game.
In live games a professional dealer deals, and the button only marks seat order. Example. In Hold'em the dealer takes no part in the contest, but in blackjack the dealer is your opponent.
At the table
At a poker table, the dealer deals the cards, collects the bets and pushes the pot to the winner. In blackjack, the dealer plays their own hand by fixed rules against the players.