A duvet is always a comforter, but a comforter is not always a duvet- so its no wonder people get confused by the terminology. To be precise, a comforter is any bed covering with two layers of cloth that contain a filling such as down, feathers, wool. cotton or a synthetic. A duvet cover is a comforter whose filling is specifically down or feathers.
A comforter usually has a decorative design printed directly on the fabric where a duvet is usually a plain white cotton fabric. One can protect either one with a comforter cover or a duvet cover.