Comets have always intrigued: they arise without warning and are often visible to the naked eye. They do not have the appearance of a planet and do not always return periodically close to Earth. They can disappear as they loose material at each passage near the Sun. They have very elongated orbits (high eccentricity), go beyond Jupiter or Saturn and then back to the Sun where they heat up and glow.
Comets are small bodies (usually smaller than 3 km) without notable gravity, and the Sun makes them evaporated. This is what causes a spectacular "tail or coma". They are formed of ice, snow and mixed rocks.