This is how we call things where I come from -
I think a comforter is stuffed all plump with some really fluffy batting or down or 'feather-alternative'. It has wide-spaced channel quilting and cross-sewing to hold the stuffing in place. It is often so decorative no bedspread is necessary.
I think a quilt is fairly flat, usually cotton, with a backing fabric and usually with a flat batting in the middle, densely sewn in some pattern or other or just hatching, and not thick and puffy at all. It doesn't need to be covered by a bedspread and it is very often made of lots of little pieces of fabric (in my case perfectly good fabric cut into hundreds of little pieces!), or possibly scraps of old clothing, joined in a decorative pattern.
Bedspreads can be a any fabric, even brocade, can be rectangularr or tailored to bed size, and some have a quilted silky-feeling backing to them, but they are still designed to COVER UP the bedclothes, not to keep you warm.
A duvet is one of those plain white comforters stuffed with goose down or 'alternative' and it has a duvet cover. I call it a comforter with airs!
JMHO!