A comforter is usually filled with synthetic fiber filler. The filler is stitched to the comforter with usual patterns on it. When you need to wash it, you need to put the whole comforter in your washer. If your washer is too small for your comforter, you would need to take the comforter to dry clean.
A duvet is usually made by more expensive down feather and it needs to be used with a duvet cover.
A duvet cover is a protective cover that has a long opening (for example, a zipper) so you can insert your feather duvet to this cover. A duvet cover is washable and will protect the duvet from stains, damage, and dirt.
In other words, a duvet cover is like a pillowcase that protects the pillow. When you want to change the look of your bedding, you can just purchase another duvet cover and replace the old duvet cover with the new duvet cover by opening the zipper on the end.
A duvet is a type of bedding consisting of a soft flat bag filled with down, feathers, wool, cotton, silk or a synthetic alternative, and typically protected with a removable cover, analogous to a pillow and pillow case.
A duvet cover is a cover for the duvet. The duvet cover protects the duvet during use. Duvet covers frequently have a decorative function on the bed, allowing for change of pattern or design for different occasions, or to serve different functions; for example, a heavier duvet cover may be used during colder seasons.
There is a difference between a duvet and a duvet cover. They are not the same thing. Just like a pillow and a pillow case.
If the item just says duvet cover, it means only the duvet cover is provided and NOT the duvet.