The Kuiper Belt is a ring of asteroids that orbit beyond the orbit of Neptune, at a distance of some 30 to 50 astronomical units from the Sun. It is believed that most of these asteroids are largely composed of ice and volatile material, rather than rock or metal like the asteroids nearer the Sun.
Occasionally, planetary perturbations make an object in this belt change its orbit so it comes much nearer the Sun. It is believed that this is the origin of most short-period comets.
The existence of this belt was hypothesided by Gerard Kuiper in 1951. A similar idea came somewhat earlier (1943) from Kenneth Edgeworth, so the belt is sometimes called the Edgeworth-Kuiper Belt. The first object in this belt to be discovered was Pluto in 1930, but its existence was not definitely established until 1992.