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A reflection nebula is a cloud of dust that is scattering EMR from one or more nearby stars, making it glow enough to be seen as a bright nebula rather than a dark nebula (dark region on the sky). It is what would be a dark nebula, but is illuminated from in front. The reflected light is from Rayleigh scattering, which occurs more at shorter wavelengths and such reflections viewed in visible light tend to appear blue.
A cometary nebula is a reflection nebula that resembles a comet, and represents one of the ways some reflection nebula can appear from our point of view. In the classical case, it is a diffuse nebula lit by a star in such way that it resembles a comet with tail.