A cloud of interstellar gas that glows by the light of emission lines. The source of excitation that causes the gas to emit may be radiation from a nearby star, or heating by any of a variety of mechanisms.
nebular gas and dust that glows as a result of atoms being excited by high energy radiation from hot, massive, young stars.
Emission nebula create their own light. Some stars emit so much light and radiation that they actually cause nearby nebulae to glow with energy, much in the same way that the electric current in a flourescent light causes the gas inside to light up. Hydrogen in these nebulae gives off a red glow, while oxygen appears green. You can find more information at The Columbia Encyclopedia. Be sure to check out some spectacular Hubble Space Telescope photos of nebulae.