21 is a casino game that is played in basically every casino and uses just one 52 card deck. There is a min of 1 and a max of 7 players playing against the dealer. The dealer takes all the cash and exchanges them for chips, allowing players to make bets, deals players cards and plays the house hand.
There are basically two ways in blackjack that a player can beat the dealer. Players can keep drawing cards to get as high as they can without going over 21 or they can let the dealer keep drawing cards until he/she goes bust. The aim of the game is to beat the dealer, so playing at a casino that is legitimate is important.
Jack, Kings and Queens are all worth 10 points in blackjack. Remember though; the Ace can be used as 1 or 11, to make up the best hand. All of the other cards are worth face value (2 spades = 2 points etc).
Usually a blackjack table will come in the form of a semi-circle and will consist of anywhere from 5-7 seats. If there are a couple of these tables grouped together in a casino, then this is known as a pit. Usually the pit will be watched by casino staff; called pit bosses.
Blackjack tables that are in online casinos tend to look exactly the same as those in land based casinos. Some of the up market casinos actually allow players to use a live dealer, so they can chat as they play and physically see the dealer to ensure the game is not a fraud. Free blackjack is sometimes available so people can get used to online game play if they need to.