The first player starts by counting to a number up to 3 (e.g "one, two"). The second player carries on counting, adding up to, or including three consecutive numbers at a time (e.g "three, four, five"). This continues until someone reaches the number 21. At this point they lose.
The name of the game is to beat the Bank, in the person of the Dealer, without exceeding 21. Otherwise you lose your bet. If you reach a Blackjack (21) your bet is multiplied by 3. If you beat the Dealer without reaching 21, you win double your bet.