A comet is a mixture of rock and dust, frozen gases and originates from the Kuiper Belt out past Jupiter. A comet is distinguished from an Asteroid by its Coma or its Tail. A Coma is a thin fuzzy temporary atmosphere that is formed when the comet passes close to the Sun. The Tail refers to the streaming gases and dust that illuminate from the ionization.
Both ISON & PANSTARRS have the ability to pack a lot of shock & awe or fizz out. But then again these two comets have the potential to be the brightest comets of our lifetimes!