There are many opinions about the nature and cause of hauntings. Here are a few common definitions and terms to describe hauntings.
Residual Haunting - a vision of an event, sound, smell etc. that is witnessed repeatedly in exactly the same way, order or time. Like a movie being played over and over.
Intelligent or Classic Haunting - a human spirit that has decided to stick around. This can be to complete unfinished business, to stay close to a loved one or due to fear or confusion. They may manifest as cold spots, shadows, electrical disturbances, moving object and even full blow apparitions.
Poltergeist – This usually manifests as movement of objects, noises, physical attacks and fires. They are frequently temporary (lasting only a few months) and revolve around children, adolescents and troubled adults.
Non-human or Spirit Haunting - VERY RARE! These can be guardians of a place or family. This can also be non-human entities harassing or attempting to "live" through attachment to humans - some people may call them demons or devils. Seeing other human's spirit having out-of-body experience or seeing your double (doppelganger or fetch) would also fall into this category.
Thoughtform - Although not a true haunting, this can easily be mistaken for one. A thoughtform, is when a person intentionally or unconsciously manifests an independent entity often through mental obsession, desire or repetition of thought. These entities tend to dissipate quickly once the human stops "powering" them.