If a player's first two cards are an ace and a card with a value of 10 (such as a 10, Jack, Queen, or King), this is called a "blackjack" and it is the highest hand in the game. So, in the context of the game, a blackjack does beat a hand with a value of 21 that is not a blackjack.
A two-card hand counting 21, necessarily consisting of an Ace and a 10-card, is a "natural", or, in the British private game, a "pontoon". Some also call it a blackjack, thogh, strictly speaking, this originally denoted the Ace of spades plus either of the black Jacks.