The difference is in a dictionary, Annacdote. Note that there are differences between AE and BE. comforter and duvet ... I can't really find the difference. As the definition of duvet is "a quilt, often with a removable cover; comforter", they, to me, seem like synonyms. More on forum.wordreference.com

Quilt - thin bed cover with some type of batting stitched between 2 layers of fabric Comforter - thicker than a quilt. It’s fabric stuffed with some type of insulation like cotton, wool, silk down, or synthetic. The outer fabric is decorative or colorful Duvet - similar to a comforter, except it will be plain white with no embellishments. It’s meant to be used with a removable duvet cover than can be laundered regularly. More on reddit.com

Quilt: "sandwich" made of 2 layers of fabric, the top layer of which is often decoratively pieced from several fabrics, with something fluffy in between, held in place with stitching in decorative patterns. Typically goes only to the bottom of the mattress. Bedspread: Bedcovering that goes all the way to the floor, often with extra length at the top to cover the pillows. Can be quilted or jacquard woven or plain cloth with embroidery. Coverlet: like a bedspread, but about the size of a quilt. Typically jacquard woven. Comforter: Like a quilt, except made from two solid (not pieced) pieces of fabric, with thicker fluffy filling than a quilt, held in place with stitching here and there. Duvet: Like a comforter, except not decorative; often much fluffier than a comforter and filled with down or feathers. Not intended to be used alone, but inside a duvet cover, sort of like a giant pillowcase, which can be removed for laundering. Blanket: Single layer of fluffy woven material used for additional warmth. May have a decorative design but is not intended to be shown. More on reddit.com

Comforter: Thickly filled blanket, doesn’t require a cover. Duvet: This is an insert, and requires a duvet cover which you purchase separately and usually comes with two matching shams. (The word is French for down but it can be filled with feathers, wool, silk, poly, etc.) Bedspread: A coverlet that isn’t as thick as a comforter or a duvet, but is long enough to drape over the sides of the mattress with overhang on at least 3 sides. Bed throw: Decorative blanket that is often seen folded at the foot of the bed, not big enough to cover the whole bed. Additionally you’ll need a sheet set (or a fitted sheet and pillow slips at a minimum), duvet cover if you decide on a duvet, and pillows. My personal preference is duvet with cover. You don’t have to wash the duvet itself as often (have it cleaned at the dry cleaner if it’s made of down!), only the cover. More on reddit.com