A comet is an object orbiting the Sun that displays a visible coma (a fuzzy surrounding region which is the comet's atmosphere) and possibly a tail, on the side away from the Sun. They are icy, i.e., made of substances solid but that can be turned to gas at temperatures reached in the inner part of their orbit (i.e., volatile material substances), creating the coma and possible tail. Classically, they have long orbital periods (tens to thousands of years) that are highly eccentric so a brief portion of the orbit is visible and displays the coma and tail effects (active comet). These are from gas and dust released as it approaches the Sun, generally from around the time it crosses Mars's orbit. The coma and tail consist of plasma pushed directly away from the Sun by the charged solar wind (gas tail), dust pushed away somewhat less by radiation pressure and not directly in the comet's shadow (dust tail), and in some cases a neutral tail of neutral atoms/molecules. A cometary hydrogen cloud forms around the comet during its active phase.
At the far end of their orbits, they move slowly such that they remain in that region for a very long time, and it is thought that there are always many long-period comets are in that state, which together are known as the Oort Cloud.
The solid part is known as the comet's nucleus. Inspection has revealed rocky surfaces. Volatiles such as water, carbon dioxide, carbon monoxide, methane, and ammonia make up the ice, which may be inside.
Two well-known comets are Comet Halley (Halley's Comet, recognized in 1705 by Edmund Halley as orbiting approximately every 76 years) and Comet Shoemaker-Levy 9 which was observed colliding with Jupiter in 1994.