>>>What's a duvet?
>>>
>>A duvet, or "continental quilt" is a quilt, that can be of varying TOG value, that covers the whole bed, not tucked in, that we put inside like a large pillow case (that matches the sheet and pillow cases) and sleep under it - no blankets or any of that restrictive, messed-up-by-the-morning stuff. All it needs is a quick shake and the bed's made. It can be down- or fibre-filled
>
>I'm still not getting it. Explain the pillow case part again. Is the case on the inside or the outside? Is this thing like a sleeping bag or like a comforter with the pillow case wrapped around it?

Duvet: It's more like a fluffy, lightweight, cotton-covered quilt, usually white, filled with down or fibre. It drapes over the edges of the bed.

Duvet cover: Imagine a very large pillow case, big enough to hold the above. You can do without it but the duvet would get soiled, for similar reasons for which you use pillow cases. The cover is like 2 sheets stiched together at their edges.

This has been very interesting to discover that, far from just not using them, many N Americans don't even know what they are.