A cover, like a pillowcase, to protect and beautify a comforter, often down, because down comforters are expensive, precious, and delicate. A down comforter with a tear, rip, or hole will leak.
After spending $300 on a down comforter, an $80 duvet is a sound investment
The ultra soft, ultra warm, non flight feathers found in ducks, geese, and other fowl, used in pillows, comforters, jackets, and other items where warmth and lightness are important. Down is measured in fill power.
Bedding/blanket comprised of two layers of cloth, filled with stuffing of some kind, and sewn together. Filling can be down, which is warm, luxurious, lightweight, and warm.
It was cold last night, so I got out the comforter