I have heard the term applied to several very different sounding voices. All female, such as Suzanne Pleshette and Mariella Frostrup.

Does it mean just low and husky? Does it mean gravelly which is kind of hard or raspy which is soft? Is it considered a good thing or a bad thing in a singer?

I would like a technical definition but cannot find one on the interwub.   

Some sound samples would not go unappreciated.