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 with a machine tool, pressed or forged into the barrel of the weapon. The grooves are the spaces that are cut out, and the each of the ridges sticking up is a 'land'. These lands and grooves can vary in number, depth, shape, direction of twist ('right' or 'left'), and 'twistiness' (turns per unit of barrel length).

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

Note that not all guns/weapons/arms are rifled. Shotguns, for instance, are smoothbore weapons. Most muzzleloaders (like a flintlock) don't have rifling. Rocket propelled grenades are an aimable launching platform for a small rocket, and have no rifling.

Typically, the inside of the barrel is of a smaller diameter than 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.

Rifle bullets are typically made of a soft metal for this reason. Their surfaces are meant to conform to the shape of the rifled barrel. Other bullets are meant to obturate, or expand to fit a barrel which is slightly larger than the projectile. The failure of a bullet to conform to the shape of a barrel in these two cases would result in dangerously high pressures inside the barrel, perhaps resulting in a 'breach' (explosion of the gun barrel); or 'blow-by' (where combustion gases leak past the bullet and down the barrel ahead of the bullet).

History

The history of rifling a barrel is covered in depth in the Wikipedia article Rifle.

Recent Developments

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.