Comets are small, fragile, irregularly shaped bodies composed of a mixture of
non-volatile grains  and frozen gases. They have highly elliptical orbits that bring them
very close to the Sun and swing them deeply into space, often beyond the orbit of
Pluto.  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. Usually a small, bright nucleus (less
than 10 km in diameter) is 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
over half of their material is ice. When a comet approaches within a few AU of the Sun,
the surface of the nucleus begins to warm, and volatiles evaporate. The evaporated
molecules boil off and carry small solid particles with them, forming the comet's coma
of gas and dust.

Once in a while we will be graced by the presence of a comet that is visible form our
night sky.  I was fortunate enough to be able to image a few from my backyard
observatory.  I have posted images of them here below: