As a rule between one and six decks of cards may be used depending on the Casino and whether you are playing a single player or multi player game. The rules specify that two cards are dealt face up to each player. The dealer deals them self two cards, one face up and one face down. Card face values apply except for picture cards which are valued as 10 points and Aces that are either valued as 1 or 11 points. A "Blackjack" score is an Ace and any 10 point card. A push results if both player and dealer have a blackjack or both go 'bust'. Players also win if their hand scores more than the dealer but does not exceed 21, in which case even money is paid. If 21 is exceeded your are deemed to have gone 'bust' and you lose.
Should the first of the dealers two cards (the one face up) be an ace the dealer will check for "Blackjack' and ask if any one wants to insure their hands against a "Blackjack" score, for even money, to protect against a possible push result should the dealer have "Blackjack" too. Only after all players have decided whether to insure or not will the dealer check the second card (the one face down). Online dealers do not check for "Blackjack" consequently if the dealer has "Blackjack" you lose all your wager (including your double and/or split bet)
Depending on the card values you have you may decide to