The Ringwraiths -- What were they?

[Mithadan]

Better topic than it sounds. We know the Ringwriths were men bound by their rings to Sauron. As I understand it, Sauron held the rings themselves. Presumably, they became "stretched thin" like Gollum and Bilbo were, until they entered the wraith world, yet they still occupy the real world to some extent. What is the wraith world? I guess it is the "plane" were spirits exist separate from a physical existence. So in a sense, Obloquy is probably correct. The Nazgul are more like the spirits of dead men than living men.