Most become visible, even in telescopes, only when they get near enough to the Sun for the Sun's radiation to start subliming the volatile gases, which in turn blow away small bits of the solid material. These materials expand into an enormous escaping atmosphere called the coma, which becomes far bigger than a planet, and they are forced back into long tails of dust and gas by radiation and charged particles flowing from the Sun. Comets are regular members of the solar system family, gravitationally bound to the Sun. It is the very fact that they are thought to be composed of such unchanged primitive material that makes them extremely interesting to scientists who wish to learn about conditions during the earliest period of the solar system. Comets are very small in size relative to planets.
Planets are usually more or less spherical in shape, usually bulging slightly at the equator.
Comets are irregular in shape, with their longest dimension often twice the shortest. The best evidence suggests that comets are very fragile. You could take a big piece of cometary material and simply pull it in two with your bare hands, something like a poorly compacted snowball. Comets, of course, must obey the same universal laws of motion as do all other bodies. Where the orbits of planets around the Sun are nearly circular, however, the orbits of comets are quite elongated.