This is an image of Comet Hyakutake taken at the end of March of 1996, when the comet was close to the Sun. Comets are composed mostly of frozen gas with bits of rock. Most comets' orbits are extremely eccentric and can take as short as 76 years (like Halley's comet) or as long as tens of thousands of years. As a comet nears the Sun, more of it starts to burn off, causing the comet to become more visible and forming its tail.
Some Astronomers believe that comets are remnants from the construction of the Solar system. Others feel that some comets resulted from the supposed explosion of a planet that formed the asteroid belt. Some also postulate that some comets are from outside the solar system and have been trapped by the gravitational forces of the solar system.
In Astrology, comets are considered to be extremely malefic.
By the way, the object Chiron has been determined to be comet and not an asteroid as previously thought.