Kuiper Belt  

The Kuiper Belt is named after Gerard Kuiper who suggested in 1951 that some comets, which pass by the sun periodically (short-period comets), and approach the Sun from within the same plane as that of the other planets, may originate in a region closer to the solar system than the Oort Cloud is.
Starting in 1992, astronomers have become aware of a vast population of small bodies orbiting the sun beyond Neptune, in a region that is much alike the one supposed by Kuiper. Observations show that these objects are mostly confined within a thick band around the ecliptic, occupying  a ring surrounding the sun in the range situated from 35 to 100 AU from the Sun.

A representation of the Kuiper belt (in scale)

This ring is generally referred to as the Kuiper Belt and it is believed to be a reservoir of source of the short-period comets, in  the same way that the Oort Cloud acts as a reservoir for the long-period comets. By now, almost 31 bodies of this belt have been found, and Pluto with its unusually elliptical orbit, is now considered to be the largest of these objects.
It is estimated that there are at least 35,000 Kuiper Belt objects greater than 100 km in diameter, which is several hundred times the number (and mass) of similar sized objects in the main asteroid belt. These objects are usually very small (10-50 km across), not very bright and with a very slow motion (it takes hundreds of years for these objects to complete an orbit around the sun). For these reasons detecting them is difficult.  
The Kuiper Belt also holds significance for the study of the planetary system on at least two levels. First, it is likely that the Kuiper Belt objects are extremely primitive remnants from the early accretional phases of the solar system. In fact, according to this theory, the inner, dense parts of the pre-planetary disk condensed into the major planets much faster than the outer parts could have done (probably within a few millions to tens of millions of years). In the outer parts of the disk - and so, in the Kuiper Belt- accretion progressed much more slowly and only very small objects were formed.
 

The Centaurs family

Occasionally the orbit of a Kuiper Belt object will be disturbed by the interactions of the planets in such a way to cause the object to cross the orbit of Neptune. It will then very likely have a close encounter with Neptune, that will send it out of the solar system or into an orbit crossing those of the other planets. These objects with unstable orbits and a future fate that cannot be foreseen are called the Centaurs family. Presently there are only nine known objects orbiting between Jupiter and Neptune belonging to this class.
The Centaurs family presents at the same time properties of asteroids and comets. In fact, only some of the Centaurs show some cometary activity (their images are a little fuzzy indicating the presence of a diffuse coma). The largest object of the Centaurs family, and the first one to be discovered in 1977, is Chiron which is about 170 km in diameter, 20 times larger than Halley. Chiron's last perihelion was on February 1996 and has a period of 50.7 years. If it ever is perturbed into an orbit that approaches the Sun, Chiron will turn into a truly spectacular active comet.