A comet, Latin for hairy star, is a large ball of ice and rock often described as a "dirty snowball." Most comets are hidden from our eyes, far from the center of the solar system in one of two reservoirs: the Kuiper Belt or the Oort Cloud.
The Kuiper Belt lies outside the orbit of Neptune, at a distance from the sun of 30 AU to 100 AU, and contains an estimated 35,000 objects with diameters greater than 100 kilometers (60 miles). The first Kuiper Belt object was detected in 1992. Most periodic comets—that is, those comets that orbit the sun on a regular schedule—come from the Kuiper Belt.
The Oort Cloud is much farther away, up to 50,000 AU from the sun. There are likely billions of comets in the Oort Cloud, but they are too small and too far away for us to detect directly. The Oort Cloud is the source of non-periodic comets.
Non-periodic comets are comets that swing around the sun once and are flung off into space, never to return. Some of these comets might, in fact, turn out to be periodic, but with cycles of a million years or more!