Generally, small bodies found in the Region in the outer solar system beyond Neptune's orbit that contains billions of small, icy planetesimals from the original protoplanetary disc that failed to coalesce into planets. The Kuiper Belt extends from Neptune's orbit at 30 AU to ~55 AU. It is ~20x wider and 20-200x more massive than the. The largest Kuiper Belt Object (KBO), Eris, is <3000 km in diameter.
Image source: http://en.wikipedia.org/wiki/File:EightTNOs.png