Who were the Nazgul?

[The Might]

Only wanted to add that, as far as their ranks are concerned, they are based on their link to the Ring, and not on their previous status in a certain society. For example it is said about the Witch-king that he was always most ready to sense the presence of the Ring. Second to him in this was Khamul, who was also the one to be most disturbed by sunlight. So clearly their qualities differed, perhaps creating a certain hierarchy.
About your descendants question...I don't know any answer either. Could be, but considering this shroud of mystery around them, I doubt that such things would have been known.