Card values in online blackjack
In a typical game of blackjack, all 52 cards in a regular deck are used. These feature four different suits (Spades, Clubs, Hearts and Diamonds), each with 13 cards. Blackjack is quite simple in not distinguishing between the four suits when assigning a value to a card. All three face cards (Jacks, Queens and Kings) also have the same value - ten. The numbered cards (2 to 10) in a deck have values matching the number on the card. This means that in any deck of 52, 16 of these cards have a value of ten. This is just over 30% of the total number of cards in the deck.
The only card that can be confusing for the novice blackjack player to count, is the Ace. That said, all aces in the deck carry the same value. This value is either 1 or 11. The choice is yours as the player. However, the choice is implicitly made for you, if by counting the ace as 11, your hand's total will go over 21. A general rule is that an ace is worth 11 if that does not lead to your hand going bust. Otherwise, the ace is only worth 1.
So, even as you are playing the game, and are asking for more cards, the value of the ace can be toggled between these two options, to improve your position in the game. This flexibility that aces thus lend your hand's total, is why aces are known as 'soft cards', and also have a set of rules not extended to the other cards in the deck.
When you are playing blackjack online, the dealing ends if the total value of your cards is exactly 21. Even if you mistakenly ask for an additional card, the dealer cannot give you one. Drawing an ace, and any one of the face cards, or the "10" card, is the quickest way to get to this total.
Historically, an extra pay-out was made to players who had drawn the Ace of spades along with a Jack of spades as a 'natural'. This extra payment for these two cards, both black and considered special, is where the game gets its name. In virtually every modern game though, no extra payment is made, and such a pairing is counted just as a regular '21'.
Many strategies have been employed by mathematically minded players to count the cards as they are being dealt, to help them compute when the odds of the cards left in the deck swing against the dealer and in favour of the player. Casinos employ several strategies to try and discourage this behaviour, including using more than one deck in any game of blackjack, although this is rare in online games. In most modern casinos therefore, it is common for most blackjack games to be played with two decks (104 cards) shuffled in together.