Cometary nuclei originate in either the Oort cloud (long-period comets) or the Kuiper Belt (short-period comets) and have been perturbed into plunging orbits by gravitational influences. Some have their perihelion so close to the Sun (sun-grazing comets) that they pass through the Sun's atmosphere. They consist of ice and rock (they are commonly thought of as 'dirty snowballs') and carry organic material, which may have played a role in the development of life on Earth.
Although many comets visit the inner Solar System every year, very few put on a spectacular show or receive much publicity. The two most recent 'great comets' (bright comets) were comet Hyakutake (1996) and comet Hale-Bopp (1997), and two other very famous comets are comet Shoemaker-Levy 9, which broke into pieces and collided with Jupiter in 1994, and comet Halley, visible 10 years earlier in 1986.
In recent years, space missions have been launched to rendezvous with various comets. The spaceprobe, Giotto, was the first to investigate the nuclei of comets from close range, imaging comet Halley in 1986 and comet Grigg-Skjellerup in 1992. Then, in 2005, the Deep Impact mission revealed what was actually inside a cometary nucleus by impacting the nucleus of comet Tempel 1, and excavating a crater in the surface.