The word "duvet" comes from French; it means "down," or the fine, fluffy feathers of birds such as geese and eider ducks.
In English, "duvet" usually refers to a comforter filled with either down or polyester batting. A "duvet cover" is like a giant pillowcase for the duvet.
A duvet's function is the same as that of other types of blanket---to keep you warm in bed. However, a duvet is usually used with a duvet cover instead of a top sheet (as a comforter is).
Duvets are made with different types of down, or batting made from polyester, cotton, or silk. You can also choose the loft (thickness) and baffle construction that you prefer.
Using a duvet without a top sheet or bedspread means it is easier to make the bed in the morning. The duvet cover comes off for easy washing; the duvet itself only needs to be washed once a year.