The Ringwraiths -- What were they?

my opinion of the nazgul was always that, while dead, they were bound to this world, much like Sauron himself, by the power of their rings. Thus, while their rings exist, and the One Ring exists, they were unable to pass into the unseen world. when the one ring was destroyed, the other rings ceased to have any power, and they could thus pass on to the unseen world, without any of their spirit remaining.