Nebula is a thick cloud made up of gas, dust and helium. Earlier, nebula was the term used to refer any extended celestial body like the galaxy. Nebulas are formed during the formation and deterioration of stars while some of them are formed by supernova explosion. With reference to the brightness and luminosity of the nebula, they are classified as planetary nebulas and diffuse nebulas. Planetary nebulas, as the name suggests, resembles the planets. They are made up of material left over during the star formation. Diffuse nebulas are those nebulas which are extended and do not have a definite boundary. These nebulas are in turn classified as emission nebulas, reflecting nebulas, and dark nebulas, depending on the cause for their luminosity. Emission nebulas are made up of ionized gas like hydrogen. Hence they are also called HII region, where HII refers to ionized hydrogen. There are nebulas which do not produce light on their own, but reflect the light from other luminous bodies like stars. They are called reflecting nebulas. Sometimes the nebulas will not have any nearby star to reflect its light. Hence they appear as a dark cloud called dark nebulas. These dark nebulas may also be found in conjunction with emission nebulas and reflecting nebulas. Nebulas can be easily observed using moderate aperture scopes.