In Blackjack, the dealer must play his hand in a specific way, with no choices allowed. There are two popular rule variations that determine what totals the dealer must draw to. In any given casino, you can tell which rule is in effect by looking at the Blackjack tabletop. It should be clearly labeled with one of these rules:

"Dealer stands on all 17s": This is the most common rule. In this case, the dealer must continue to take cards ("hit") until his total is 17 or greater. An Ace in the dealer's hand is always counted as 11 if possible without the dealer going over 21. For example, (Ace,8) would be 19 and the dealer would stop drawing cards ("stand"). Also, (Ace,6) is 17 and again the dealer will stand. (Ace,5) is only 16, so the dealer would hit. He will continue to draw cards until the hand's value is 17 or more. For example, (Ace,5,7) is only 13 so he hits again. (Ace,5,7,5) makes 18 so he would stop ("stand") at that point.

"Dealer hits soft 17": Some casinos use this rule variation instead. This rule is identical except for what happens when the dealer has a soft total of 17. Hands such as (Ace,6), (Ace,5,Ace), and (Ace, 2, 4) are all examples of soft 17. The dealer hits these hands, and stands on soft 18 or higher, or hard 17 or higher. When this rule is used, the house advantage against the players is slightly increased.
Again, the Blackjack dealer has no choices to make in the play of his hand. He cannot split pairs, but must instead simply hit until he reaches at least 17 or busts by going over 21.