The objective is very simple: get the value of your hand closer to 21 than the dealer's without exceeding it (going bust).
Blackjack - Wikipedia http://en.wikipedia.org/wiki/Blackjack
Very elaborate guide about blackjack that begins with some background information and proceeding with rules and strategies for playing the game.
Blackjack – Odds and Probabilities http://www.wizardofodds.com/blackjack
Advanced strategies for playing blackjack discussing insurance, factoring the house edge before deciding your move and plenty more.