Black's move prevents white from advancing a pawn to d4, and starts the fight for the center. It is the most popular response to a king's pawn opening (with e5 a close second).
This is the standard response to black's move, and is used by about 75% of players. Many other responses are possible, and are known as "anti-Sicilians".
There are several variations for the next move, including the Classical (Nc6), the Schveningen (e6), and the Dragon (g6). We will be playing the Najdorf variation (a6):
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The purpose of moving the pawn to a6 is to prevent white's bishop from moving to b5, which puts black's king in check. It also prevents white's knights from moving to this square, and lets black move a pawn to b5 at his or her leisure.
Black's next move is usually to advance the king's pawn to e5 or e6, which sets up a good pawn position and good leverage on the center of the board. In general, Black's plan is usually to start a minority pawn attack on the queenside and put pressure on White's e4 pawn. Often this can be done through playing ...b5, ...Bb7, and putting a knight on c5. White also has to look out for exchange sacrifices by Black on c3 where White usually has a knight posted guarding the important e4 pawn. This exchange sacrifice is a recurring theme in the Sicilian Defence.
At this point, white has several possible strategies:
Pin black's knight by moving the bishop to g5, and then exploit the pin by advancing the pawn to f4.
Queenside castle, moving the bishop to e3 and the queen to d2. This is called the English attack. The pawns on the right should be advanced to f3 and g4.
Kingside castle, moving the bishop to e2. This sets up a quieter, more positional game.
