A comet is a small body, usually consisting of ice and rock, in the solar system that orbits the Sun and occationally exhibits a tail.
Comets' orbits are constantly changing. Some are moved into the Suns orbit that destroy the comets, while others are thrown out of the solar system forever.
Comets also can contain amino acids, one of the building blocks of life. So some scientists think that colliding comets may have brought the first ingredients of life to Earth.
Most comets are believed to originate in a cloud at large distances from the Sun consisting of debris left over from the condensation of the solar nebula (cloud of gas or dust). The outer edges of such nebulae are cool enough that water exists in a solid state.
Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
The word comet came to the English language through Latin cometes. From the Greek word komē, meaning "hair of the head," Aristotle first used the derivation komētēs to depict comets as "stars with hair."