Comets have been known since antiquity, unlike the other small bodies in our solar system. In 240 B.C. The Chinese recorded the appearance of Halley Comet. The appearance of the same comet during the Norman Conquest of England in 1066 is depict in the famus Bayeux Taperstry.
878 comets have been cataloged and their orbits are roughly calculated as of 1995. 184 of these are periodic comets, who;s orbital periods are less the n 200 years. Probably some of the other are periodic also but their orbits are not establish.
Comets are referred to as "dirty snowballs" or "icy mudballs". They are a mixture of ices, dust, water and gases. They are for some reason teh reminder that did not get incorporated into planets when the solar system was formed.