A comet is an icy solar system object that, when passing close to the Sun heats up and begins to spew out gasses outgas, displaying a coma or tail. These effects are due to the effects of solar radiation and solar wind on the center of the comet.
Comet centers (or nuclei) range from a few hundred yards/metres to tens of miles/kilometres across and are composed of loose collections of rocky particles, ice, and dust. The tail is much larger, stretching generally from 600,000 to 6 million miles ! Many comets are large and bright enough to be seen from earth.
Comets have a wide range of orbital periods (length of an orbit around the Sun) ranging from several years to several millions of years. Their orbits are very unique in that they approach very close to the Sun — creating their spectacular celestial “streaking” images.
Short-period comets originate in the Kuiper Belt which lies beyond the orbit of Neptune. Longer-period comets (like ISON) are thought to originate in the Oort Cloud, a cloud of icy bodies extending from outside Pluto to halfway to the next nearest star.
Comets are distinguished from asteroids by the presence of an unbound atmosphere surrounding their central core. This atmosphere has parts termed the coma (the central atmosphere immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun’s light pressure or outstreaming solar wind).
As comets orbit and pass close to the Sun, they lose some of their ices and dust, and over time can be seen as becoming “extinct” and resemble small asteroids. Asteroids have a different origin, having formed inside the orbit of Jupiter rather than in the outer Solar System.