Speed: Bullet to Correspondence Rating:
Sicilian Defense: Najdorf VariationBETA
a4 stops black from the desirable b5 expansion. However, it almost permanently weakens the b4 square, and does not develop a piece, so it's certainly not the best move in the position.
The Najdorf Sicilian is one of the most popular variations in all of chess. It is highly theoretical, and leads to extremely double-edge positions with winning chances for both sides. White's lead in development means they can often build up a huge kingside attack, while black has lots of counter-attacking chances on the queenside due to their solid center and flexible pawn structure.