Comets travel from the outer reaches of the solar system to the sun. A comet is actually an icy lump that develops a shining head and two tails when it is warmed up by the sun. A solid nucleus of snow and dust inside the gas cloud makes up a comet's coma. The coma thins out behind head, and the dust and gas streaming from a comet form two tails. Those tails can stretch for millions of kilometers.
Comets can be divided into two types according to their sources: Long-period comets and Short-period comets. Long period comets take more than 200 years to complete an orbit around the Sun and are believed to be originated from the Oort Cloud. On the other hand, Short period comets take less than 200 years to complete an orbit around the Sun and are originated from the Kuiper Belt.
The first person who realised that a same comet can regularly visit the earth is Edmond Halley, an English astronomer. The comet that he discovered is named after him - Halley's Comet. Halley's Comet made its last visit in 1986 and is believed to return in 2061 with a period of every 76 years.
Halley's Comet seen from earth in 1986