The Ringwraiths -- What were they?

As I see it, The Ringwraiths are kind of living spirits. They are ghosts and are actually dead, but the power of the Ring holds them in this world as a prisoner. When Frodo puts on the ring, he passes into the shadowworld to, so he can see the spirits they really are. As soon as the Ring is destroyed, they are no longer tied to this world and can finally rest in the shadowworld where they belong