A duvet cover fits over a duvet. You can take it off and wash it easier than a comforter. A comforter is all-in-one - just the cover for the bed without having to have anything else. The duvet cover has to have a duvet inside it.
Just to clarify though, a "duvet" is usually labeled as a comforter on the package. I was at the Pottery Barn outlet store today, and all the white "comforters" were meant to go inside the duvet covers. A true comforter I guess, is just a thick blanket meant to be the only, or top, layer on your bedding. I prefer a true comforter b/c I hate how duvets slide around inside the covers...but typically the cutest and easiest to find is a duvet...also means you can more easily change the look of your bed since a cover is cheaper than replacing the thick comforter. Hope I haven't caused more confusion!
Re: difference between a comforter and a duvet coveR?