The dealer is the only person you need to beat, so while other bettors can join you, they are all competing to beat him or her. When playing blackjack, you want to beat the dealer's hand.
They choose to 'hit' or 'stand' to get as close to 21 as possible or to hit 21 directly. The game of blackjack pits players against a dealer while both sides strive to reach a hand score of 21. Blackjack is the most popular game you'll find bettors competing in, regardless of whether they are in person or online.