NebulaA cloud of gas and dust in space. Depending on how it shines, or does not, it is called an emission, reflection, dark or planetary nebula. The word once also covered galaxies, whose nature was unknown.
Emission nebulaA gas cloud lit up by hot young stars that make it glow, like a neon sign. Its hydrogen emits a red light that photos reveal; the eye mostly sees grey or pale green. The Orion Nebula (M42), about 1,350 light-years away, is visible to the naked eye.
Dark nebulaA dust cloud dense enough to hide the stars and glowing gas behind it. It shows up in silhouette, like the Horsehead in Orion or the Coalsack near the Southern Cross. The dark rifts in the summer Milky Way are made of them.