Emission, reflection and dark nebulaes are areas of a galaxy where stars are formed from molecular hydrogen (HII). The Milky Way is littered with dust and an abundance of HII.
Emission nebula glow red because of an HII region becoming ionized by nearby stars. The electons "freed" by this ionization are continually absorbed and reemitted causing it to glow red.
Dark nebulas are cool clouds of dust and molecular hydrogen that can be seen when silhouetted against a bright background (ie. the Horsehead nebula).
Reflection nebula are areas in which material that contain dust is reflected from nearby stars. Because of the shorter wavelength of light this "dusty" area appear blue.
Stars are formed when molecular hydrogen accretes, or falls into itself, gradually increasing in gravity until it reaches a certain mass. As this material continues to fall into itself the temperature and pressure increases until nuclear fusion begins and a star is formed.
© Coates Astrophotography