Orion Nebula. Hubble Space Telescope image.
An emission nebula is a nebula that displays an emission spectrum because of energy that has been absorbed from one or more hot, luminous stars and reemitted by the nebular gas at specific wavelengths. This is different from a reflection nebula, where the light from the nebula is simply reflected light from the central star. The difference becomes clear from looking at the spectrum of the nebula and comparing it to that of the stars providing the initial energy. If it is a reflection nebula, then the spectra will match; if it is an emission nebula, it will show emission lines of its own.