A Wraith is a ghost, the soul of a person who has died unfulfilled and now haunts the Shadowlands.
For some reason, known or unknown to the wraith, they are still tied to the living world (the Skinlands) and cannot pass on to the ineffable afterworld. Wraiths are confined to the Underworld, a parallel reality which is tangentially connected to the living world, but distinct with its own laws and physics.
About 5% of people becomes Wraiths after death in time of relative calm. Far more in times of great calamity and suffering. In the World of Darkness, it is plausible a sizeable percentage instantly become Spectres at the time of their death.