Emission nebula

An Emission Nebula is a interstellar cloud of dust and high-temperature gas. The atoms that comprise emission nebulae are energized by ultra-violet light from a nearby stars, and hence emit radiation.

Reflection Nebula

A Reflection Nebula is a cloud of dust that reflects the light of nearby star(s). Unlike emission nebula, the nearby star(s) of a reflection nebula are not in close enough proximity to ionize the gas of the nebula. However, the proximity is enough to make dust visible as the light reflects off of the dust.

Reflection nebula are created when the light from a star is scattered or reflected off a neighboring dust cloud.

Variable Nebula

A Variable Nebula is a reflection nebula that gets brighter or darker because of the chances in its nearby star.

Dark Nebula

A Dark Nebula is a highly dense cloud of dust and gas which is illuminated by whatever bright object is in the background. Basically, they are obscure or block the light from the bright object behind them. Their high density allows them to scatter and absorb optical light, making them completely opaque at visible wavelengths to us.