The goal in blackjack or "21" is to have your cards total 21 or as close to it as possible without going over or "busting." Players win when their total exceeds the dealer's total, or when the dealer busts.
Cards are valued at face value, with picture cards (King, Queen and Jack) counting as 10, and Aces valued at either 11 or one, at player's choice. Players receive two cards; dealer's cards are dealt one down and the other face up. If players like their cards, they "stand," but if they want to add to their total they "hit." After players have completed play, the dealer plays in like fashion.
With traditional blackjack, when your first two cards are an Ace and a 10 card, you have a blackjack and the dealer will pay you 3-to-2 on your original bet. However, if the dealer also has a blackjack it is a stand-off. If the dealer's up card is an Ace, he'll offer you "insurance" at up to half of your original bet. If the dealer has blackjack, you'll receive 2-to-1 on the insurance bet, and lose the bet if he does not.
From time to time, casinos may offer specialty blackjack games that are subject to different rules and payoff odds than traditional blackjack. Any such rule variations and payoff odds will be posted at the subject tables.