I'm studying Astronomy in Uni at the moment so maybe I can clarify some of this
The Kuiper belt is a region in the outer solar-system where there are many large asteroids orbitting the sun, similar to the asteroid belt (which exists between Mars and Jupiter), but outside the orbit of Pluto.
As far as I know, Pluto is still 'officially' a planet, but most astronomers consider it to be a very large Kuiper belt object, rather than a planet. Its status as a planet is mainly a historical one, but if discovered today, it would almost certainly be classed as a Kuiper belt object I feel. It isn't totally spherical, it's 'moon' is almost as big as itself, and it has a highly eccentric (i.e. non-circular) orbit unlike the other planets, plus there are some other key differences I can't quite remember now. I'm not 100% up-to-date with this, since it isn't my field, but that's what the current belief is.
@JeBuS - you weren't refering to the IAU by any chance?