comet is a small solar system body made of Ice and dust.
Due to some change in gravity they travels towards Sun and when reaches near Sun the ice sublimates and form a coma and tail..Most comets come from an area called Ootr;s cloud Few comets are in orbit around Sun in smaller orbit called short period comets..Thier period id less than 200 years. comets captain all types of ice water ammonia carbon monoxide etc.Tail of a comet will be always away from Sun.The nucleus of a comet may be about 15 KIM in size.But the tail will be very thin and millions of kilometers long.
picture about space.com.