Funny, and I was told in a class not too long ago that a tied quilt is not a quilt, it's a "coverlet".
The definition Jan posted seems to sort of back that up, in that it appears a quilt is a type of coverlet?
I consider tied quilts to still be quilts. In my mind, a comforter is not pieced, nor is it bound, and it has high loft and large spaces between quilting lines. I don't think I've ever seen anything with ties that I'd consider to be a "comforter".
Funny how complex it gets, and almost arbitrary in some ways.