What is a Comet?
A Comet is a lump of icy gas and dust. They can be over 100km in diameter and exist at the very edge of our solar system in an area called the Oort cloud.
Occasionally one is dislodged from it's orbit by the passing of a nearby star and takes an elliptical path around the Sun and crosses the paths of the planets.
Hale-Bopp comet displaying its tail
When a comet gets close to the Sun it starts to melt and forms a tail, extending away from the Sun. A Comet's orbit is regular and at predictable times these tails can be seen from Earth with the naked eye.
Halley's Comet is probably one of the most well known comets. It is visible from Earth every 75 to 76 years. Halley's Comet was last seen from Earth in 1986. It will next be visible in 2061. When it appeared in 1986 several spacecraft were sent to investigate it and returned valuable data about the comet's structure.
Most comets will take thousands of years to make a full revolution of the Sun. The continual path around the inner solar system close to the Sun will cause a comet to loose much of its structure and it will eventually fade away to an insignificant amount of material.