Some information originally taken from TiredOldFart, on SKSboards.com
This question comes up semi-regularly, and in fact it is one of the very questions which caused GunWiki to be created. In order to understand the answer, there are a few underlying concepts which must first be understood.
What Is Rifling
Rifling is a series of spiral grooves, cut into the sides of a barrel from its chamber to the end. The purpose of rifling is to impart spin to the projectile, which stabilizes it in flight and greatly increases accuracy. The higher the rate of twist, the more spin is imparted to the projectile.
Smoothbore weapons have no lands or grooves. The lands and grooves only exist on rifled barrels. Although the term would seem to apply to rifles only, rifled barrels appear on a variety of weapons. Pistols, rifles, "slug guns" (shotguns designed to fire slugs only), and even certain cannon actually utilize rifling.
Lands and Grooves
In a rifled barrel, the lines you see (the slices cut out during the rifling process) are the "grooves". The "lands" are the sections of metal left untouched by the rifling process.
A rifle barrel looks like a cylinder with spiral-cut lines going down it. The rate of twist varies by the intent of the manufacturer. Twist rate is expressed in terms like "1:7" read as "one in seven". In the example above, it means that "in every 7 inches, there is one complete spiral". The first number is, as far as I have seen, always a 1. The readiest way to state this is: "For all twist rates of 1:X, each groove makes one complete revolution for every X inches of barrel length".
Brief History of Rifled Barrels
Rifled barrels first saw widespread military use around the time of the Civil War, although rifled barrels apparently date back to at least the 1500's. The purpose of a rifled barrel is to ensure that the projectile is spinning when it leaves the barrel, which imparts considerable improvement in accuracy compared to the smoothbore alternative. There is no such thing as a "smoothbore rifle", although firearms commonly described as "black powder rifles" may actually be muskets.
-- SeanNewton - 30 Dec 2005