A Spirit is the soul of a deceased human or supernatural being that has found peace, since the destruction of the Other Side, or had previously been sucked into "Oblivion". With the exception of Katherine, who was confirmed to be in Hell during its destruction, it remains to be seen what happened to the other spirits that were in Hell. Some spirits can continue to linger, however, in the Space In-between or Spectral Plane. Spirits can rarely interact with the physical plane and are believed to be unable to be resurrected through traditional means of witchcraft.
Spirits do not appear any different dead than they did when they were alive. Whenever they appear to the living, they are usually seen dressed in the very same attire they were wearing at the time of their deaths.
Powers and Abilities
Intangibility: Typically, spirits cannot physically feel nor can they be physically felt. However, unique supernatural beings, such as psychics can interact with spirits within the In-between. Bonnie, as the Anchor, could also interact with Katherine who was unable to past through her to enter the Other Side.
Species-Specific Abilities: Spirits of supernatural beings should still possess their given supernatural abilities if their spirit is summoned or they're resurrected. For example, when the the Bennett witches manifested from peace, they retained their abilities to practice magic and helped Bonnie redirect Hellfire and destroy Hell. When Josette was resurrected by the Necromancer, she remained a powerless witch since she gave up her magic to her brother.
Witchcraft: Spirits are susceptible to the forces of witchcraft, particularly necromancy. The Necromancer has a proficient understanding and workings of necromancy that allows him to see across dimensions, communicate with, raise and completely resurrect spirits that he can force to do his bidding.
With the destruction of the Other Side, souls were sucked into Hell or allowed to find peace. Prior to the destruction of Hell, spirits were described to endure physical and emotional turmoil, with literal and eternal fire and brimstone. Sybil recounts that it is where Arcadius feeds on the souls of the damned. Hell, however, was destroyed when Bonnie and other Bennett witches redirected hellfire back into the dimension with Katherine's spirit back within the dimension. Spirits that reside in peace experience something drastically different as they can watch over their loved ones both in the physical plane or with one another in peace. Josette described it as like she was in a dream where everything is warm and happy.