This is an article my friend wrote a while ago, trying to explain the various creatures, objects and places featured in Supernatural to non-watchers.
Creatures and beings
Spirits / Ghosts
• Ghosts of people who have died, are still here for: revenge, just an evil soul naturally, unfinished business
• Some spirits may only be seen by certain people – those with psychic abilities, those the spirits wish to harm or contact, or those in a certain state of mind.
• Ghosts are tied to one place and cannot leave it, usually where they died.
• Can make lights flicker on and off, move through walls, have telekinesis, teleport, make physical contact with people eg can punch them, talk to people
• The most violent ghosts can possess people and move around in their body, moving miles away from where they haunt. If they are forced to leave the body they are thrown back to where they haunt.
• Cold spots are signs of ghosts/spirits