Talk is cheap. We can tell stories all day long, but what exactly are we talking about when we say “ghosts?” Based upon sightings, these vary considerably.
The ghost as filmstrip: This kind of ghost appears like a slice of another time. It walks through walls as if they don’t exist (and maybe in his time, they didn’t. This ghost appears to be non-sentient, just a visual trace of someone from the past.
The ghost as sentient being: This ghost appears aware of those present. It may or may not communicate with them.
The ghost as messenger: This ghost may or may not be someone known to the witness, but he has a message to convey which he does telepathically, verbally, or in other ways. An example would be a ghost appearing someplace significant in a way that would be meaningful to a witness. A ghost like this may appear to someone during a crisis of some kind.
The ghost as harbinger: This kind of manifestation appears to warn us about something.
The negative ghost: This ghost doesn’t want you to be there and will make that clear. These presences as reported by witnesses can be quite terrifying.
The angelic or daemonic presence: not actually ghosts, but paranormal presences. They are self-descriptive.
Spirits of the once living: This ghost was a relative, or perhaps just a person who lived in the home before you moved there. It is usually neutral or sometimes even welcoming. Witnesses notice something’s there, even if it’s not visible. Pets do, too.
Elementals: These are an entity which was never human. They’re something else. Nature spirits (called devas by the Findhorn people), fairies, other beings fall into this category.
Spirit of some other kind: not an elemental but not a human spirit, either. Witnesses see these as shadows, among other things. Spiritualist tradition warns that these can be dangerous, but they are not daemons.
Ghosts with an agenda: These spirits have something on their minds, some purpose that propels them and some would say prevents them from “crossing over.”
Ghosts with issues: These ghosts seem to exist in a spiral of dysfunction: absorbed by issues that are as dead as they, yet held back from crossing over by those very problems. Needy ghosts (codependents) fit into this category.
Ghosts of Contagion: These ghosts appear to have a negative energy field, which can be very unpleasant to be around. They spread malaise. A type of negative ghost.
These basic classifications are just a starting place. There are more. Also, these are ghosts, as opposed to other paranormal beings: perhaps vampires, werewolves or other folkloric creatures. But it gives us something to work with as we explore ghosts in terms of psychological, spiritual and cultural symbolism.