Duvet and comforters are forms of blankets or quilts taken as protection from the cold. The differences between a comforter and a duvet lie in the material they are made of and their usage.
A comforter is a thick, quilted, fluffy blanket intended to keep the user warm. It is usually filled with synthetic fiber filler. A Duvet is a type of bedding - a soft flat bag traditionally filled with down or feathers, or a combination of both and used on a bed as a blanket.
edit Differences in Usage
While a comforter is used only as a blanket to keep one warm, a duvet is also used as bedding. It is a single piece that serves as both bedding and blanket.
While a comforter only forms one part of bed linen, it is always used to cover oneself. Other bed linen like bed sheets, bed covers, quilts etc. are needed to be used along with a comforter. Comforters go on top of the traditional sheets. In case of a Duvet, no other bed linen is needed. Duvet is a single piece that serves the purpose of both and solves the complexity of making a bed.
edit Filling in a Comforter vs Duvet
While a duvet is traditionally filled with feathers or down, these days they are also filled with silk, wool or artificial fibers. Comforters are filled with layers of material such as polyester batting, down feathers, wool, or silk. The comforter is stitched or quilted to secure the filling and keep it evenly distributed.
edit Differences in Washing and Care
A comforter is generally used as it is and hence can be washed. A duvet is sometimes placed in a duvet cover and used, hence the duvet cover is washed separately. The duvet is not washed as it is generally made of natural materials which can get damaged.
edit Size of a duvet vs a comforter
Comforter sizes correspond with bed sizes: twin, full, queen, King and Cal King. They can run slightly larger than actual bed sizes to allow for draping over the sides of the bed. It may fall around 3/4 of the way down towards the floor from the bed. Duvets are typically smaller in size and come in line with the actual bed size as it is supposed to snug on the whole bed.
edit Cost comparison
A Duvet is generally more expensive than a comforter as the former serves the purpose of bedding as well as blanket. It saves the cost of other things like bed sheets and quilts etc. So duvet is priced higher than a comforter.