Duvet: Sometimes referred to as a comforter (especially in the United States), a duvet is a type of bedding that is a soft flat bag filled with down, feathers, wool, or a synthetic alternative. It is protected with a removable cover, the duvet cover.
Duvet cover: Duvet is to duvet cover as pillow is to pillowcase. A duvet cover is a protective layer that slips over the duvet and has a closure. Because duvets and comforters can be expensive and difficult to clean, duvet covers are useful because they protect your comforter during use and are easily removed and easy to wash. With duvet covers, you can also quickly and easily change the look of your bed and room without having to completely redecorate.