Comets are balls of ice and rock that vaporise as they approach the Sun. Tails of gas form behind them, which are blown away from the Sun by solar wind.
Famous comets are Halley's Comet, which orbits the sun every 76 years, and Hale-Bopp, which takes 4026 years.
The Kuiper Belt extends from Neptune to beyond Pluto, and it contains lots of comets. Some people think because Pluto is in the Belt, it should be classified as a comet.
The Oort Cloud is a sphere of about 10 trillion comets surrounding the Solar System. It is nine trillion km from the Sun.