Having lived in both the US and the UK, a duvet goes inside a duvet cover that gets laundered. No top sheet necessary. A comforter sits on top of sheets and is decorative in its own right (rather than a duvet which just be out inside a decorative duvet cover). Comforter mostly in the US and duvet in UK.

In the USA: A quilt is two layers of cloth filled with padding held in place by hand-stitched geometric designs, often made of patches of various scrap remainders of fabrics. They generally are not very thick. Quilts are often historical items hundreds of years old back to Colonial Times and worth $$$$-$$$$$. They may also be modern-made using historical patterns and methods. A comforter is made of two layers of usually uniform cloth permanently sewn together. They are generally filled with synthetic insulation. These are modern items and the most cheaply made. A duvet is like a comforter, though made with a plain white fabric. They are usually filled with down. The duvet is slid into a “duvet cover”, a slip-cover made of a more highly-decorated, high-quality fabric. This allows using multiple covers for one duvet. They are far more expensive than comforters. I don’t know what the British think of all this. :)

I use the word "comforter" often (in the winter). The word 'duvet' is something I hear women say, but most men could not tell you what it is, haha. They are different, but I can't tell you how they are different. (This is in the U.S.) Good luck!

In the UK we say 'duvet' more than comforter! I would also use 'quilt' but would not use 'comforter.'

yep in U.S. just use comforter! Unless it really is "quilted" like an old fashioned quilt.