1. A coverlet or blanket made of two layers of fabric with a layer of cotton, wool, feathers, or down in between, all stitched firmly together, usually in a decorative crisscross design.
2. A thick protective cover similar to or suggestive of a quilt. http://dictionary.reference.com/search?q=quilt
Comforters can be as big as full-size quilts or blankets in this country. (I had never even heard of duvets before.) Quilts have more, well, quilting that comforters. (It is not easy to describe the difference.) Also, as you noted, Tdol, quilts tend to be more decorative.
Duvet, quilt, comforter, they all mean Beizi in Chinese.
What is the difference (between or among) them?
Which is most frequently used, and which is obsolete?
What defines a quilt is the quilting itself, the elaborate stitching that is used to finish the covering. You can look on the Internet to find examples. A comforter would be a large, thick bed cover, usually thicker than a blanket, and often made with down (feathers). This would cover a bed and blanket. A duvet is like a conforter, but it has a cover that zips over a comforter. A duvet often serves as the blanket and the comforter.
In my experience, duvets are more common in Europe, particularly in Germany and Scandanavia, than in the US. Quilts and comforters are both common in the US.