Let's start with an analogy. A duvet cover is to a duvet as a pillowcase is to a pillow. A duvet cover is what slips over your duvet, serving as a protective layer and the most visible part of your bedding. People opt for duvet covers due to their popularity, though there are many factors to consider prior to making a purchase. Duvet covers are machine washable, generally inexpensive, comfortable, and easy to switch out if you get bored of one style.A duvet is what goes inside of a duvet cover (makes sense, right?). The duvet, also known as an insert, is generally defined as a flat bag that is filled with either down, wool, feathers, or a synthetic alternative. Duvets cannot be used without a duvet cover, and, unlike the cover, require dry cleaning as opposed to machine wash.