There are several different types of nebulae,and there are several classification schemes. One is based on spectroscopy and the light which is seen from the nebulae:
Emission Nebulae: Emit light because the atoms in their gases are excited by high energy radiation of stars involved. They show emission line spectra.
Reflection Nebulae: Reflect light of nearby stars by their dust particles. Therefore, their spectra are the same as those of the stars, typically continuous spectra.
Dark Nebulae: Absorb light - Their gas component can be seen as absorption spectra in the light of background stars, their dust component by absorbing and reddening background light.
