The object is to have a hand with a total value higher than the dealer's without going over 21. Kings, Queens, Jacks and Tens are worth a value of 10. An Ace has the value of 1 or 11. The remaining cards are counted at face value.
In blackjack, players attempt to reach a score of 21 without exceeding it before the dealer hits 17. You can win if you don't bust and your total is higher than the dealer cards. Hitting exactly 21 can mean even bigger winnings.
