Blackjack and 21 are both versions of a single game: blackjack. The differences between the games come down to how much money the player must risk and how many cards are dealt. In blackjack, the player is dealt two cards face down, and must decide whether to keep the card or raise it. If the player keeps the card, he or she gets to draw another card.
If the player raises the card, another player is dealt a new card and the game continues as normal. The goal in 21 is to get as close to 21 points as possible before going bust. In both games, if a player goes bust (has no more cards), they lose whatever money they have left in their hand. Blackjack is usually played with less money than 21, while 21 is played with more money than blackjack.
The biggest difference between blackjack and 21 is that in blackjack, the goal is to get as close to 21 points as possible before going bust. In 21, the goal is to get as close to 00 points before going bust.
Another difference between blackjack and 21 is that in blackjack, if a player goes bust (has no more cards), they lose whatever money they have left in their hand. In 21, if a player goes bust (has no more cards), they lose all of their chips.
So while there are some minor differences between blackjack and 21, on the whole they are basically the same game. The biggest difference between them comes down to how much money players can risk and how many cards they are dealt.
