Downvoters: /msg me with your reasons.
Nobody has mentioned it yet, so I'll outline the basic technicalities:
The barrel of a rifle has curving extrusions all along it. If you can't visualise this, think of it as a pistol with a long, thin corkscrew placed in the barrel, and stretched. The basic idea is that the bullet is made to spin as it is fired.
Because the bullet is spinning around, it fires more accurately. This is because any differences around the bullet are spread around. Say the bullet was slightly less aerodynamic on one side from the explosion firing it, whe it rotates around, any path distortion should correct itself.
I may have some of the details wrong, but that's the basic idea.
UPDATE! - 13/06/2001
I am told by SkiBum5 that the spining creates a stabilizing gyroscopic effect that keeps the pointy end of the projectile forward, not tumbling end over end... which is a startlingly sensible suggestion.