Rifling

Rifling is the means by which a firearm gyroscopically stabilizes a projectile. Most rifling is created by a number of grooves that have been cut, pressed or forged into the barrel of the weapon. Typically, the barrel is of a smaller diameter than the bullet, and the rifling is of approximately the same diameter as the bullet. When the cartridge is fired, the bullet is forced into the barrel and the rifling engages the bullet, deforming it somewhat. As the bullet is propelled down the barrel, it begins to spin.

The spin imparted by rifling significantly improves the stability of the trajectory, improving both range and accuracy.

Miscellaneous Notes

The grooves most commonly used in modern rifling have fairly sharp edges. More recently, octagonal rifling has become popular, as it seems to produce better accuracy due to the fact that it does not damage the bullet as badly as conventional rifling.
The flat surfaces of the barrel between the grooves are called the "lands".