Comet structures are diverse and very dynamic, but they all develop a surrounding cloud of diffuse material, called a coma, that usually grows in size and brightness as the comet approaches the Sun. A small, bright nucleus (less than 10 km in diameter) is sometimes visible in the middle of the coma. The coma and the nucleus together constitute the head of the comet.

As comets approach the Sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the Sun. When far from the Sun, the nucleus is very cold and its material is frozen solid within the nucleus. In this state comets are sometimes referred to as a "dirty iceberg" or "dirty snowball," since about half of their material is ice. When a comet approaches within a few Astronomical Units (1 AU equals approximately 150 million kilometers) of the Sun, the surface of the nucleus begins to warm, and volatile material on the comet evaporates. The evaporating gases carry small grains with them, forming the comet's coma of gas and dust.