Comets, often referred to a “dirty snowballs,” are small bodies of loose material primarily composed of ice, dust and rock. Comets are artifacts of the formation of the Solar System and most originate from the Kuiper belt or Oort Cloud. As a comet makes it’s way into the inner Solar System the Sun’s radiation causes the ice, dust and rock to vaporize from the nucleus through a process called sublimation. The nucleus is then surrounded by a cloud of dust and gas called a coma. As the comet falls closer to the sun its tail begins to grow. Typically, the closer the comet is to the sun the more intense the radiation, and greater the loss of material from the nucleus.
Helpful Links:
IAU: Minor Planet Center – Comets
Heavens-Above list of the brightest currently observable comets.