Hi Andre,

An asteroid is a relatively small (compared to the planets), inactive, usually rocky object that orbits around the Sun. Comets are also relative small objects, but they are made of a combination of ice and rock. When the orbit of a comet takes it close to the Sun, some of the ice can vaporize, forming a tail of dust and gas that streams away from the body of comet. Both asteroids and comets are believed to leftovers from the formation of the solar system 4 billion years ago. Most asteroids are found in the region between the orbit of Mars and Jupiter that we call the asteroid belt, but some asteroids do pass closer to the Earth. On the other hand, comets are thought to originate beyond the orbits of the outer planets in the regions called the Kuiper Belt and the Oort Cloud.

You can learn more here-

http://solarsystem.nasa.g...file.cfm?Object=KBOs

http://neo.jpl.nasa.gov/faq/

Kris