Wraiths (W. "twisted Ones" or "withered Ones") were a class of Undead.They were Souls that were "bent" by dark sorcery and had become imprisoned within their mortal bodies, though these bodies had withered away and only remained visible within the shadow-realm while they had become invisible in the "outside world".
The name was also applied to "Warding Ghosts", that were bound to a specific place or artifact, guarding it and being slave or prisoner to it at the same time.
Thus wraiths were different in origin and appearance from other undead such as Wights, spirits who took dead bodies as their homes, or the Ghosts of Oathbreakers for example , who were not the product of evil sorcery but Souls that remained as incorporeal existences within Arda as they had bound their spirits to it's matter by some unfulfilled oath or other dedication.
See: