There are four basic types of nebulae:

Emmission Nebulae

Clouds of gas and dust which glow in the visible light wavelengths due to energy which is absorbed from nearby hot stars and then radiated as visible light.

Reflection Nebulae

Clouds of gas and dust which reflect visble light from nearby stars.

Dark Nebulae

Clouds of gas and dust which block the light from the stars beyond them, causing what appear to be black voids in the starfield.

Planetary Nebulae

Shock front phenomena which are caused by dying stars. A planetary nebula is a shell of expanding gas around a star that is close to the end of its life. Through the telescope it looks like a small green disc like a distant planet.

Pictures of Nebulae (click on an image to get a better view)