Poltergeist, incubus, shadow people...
We typically hear of these things as different spirits...different personalities trying to reach forth in communication. I often read on other websites or publications how to get rid of a poltergeist...or if you have a shadow person then you need to do "____". The spirit world makes so much more sense once you begin to put all the pieces together in one picture. Just as we have the Gifts of the Holy Spirit...many Gifts, yet the same spirit (1 Corinthians 12:4)...so does the demonic world function.
A location that was perhaps the scene of a tragic crime...murder, rape, etc...can have a spirit lingering there that may manifest in several ways. You may see an apparition, perhaps hear voices and screams. Another person at the same location may be driven by the residing spirit to carry on the same crime that was committed there. Objects may be thrown and other noises may be heard. It is all simply the spirit allowing itself to be know, installing fear and at the very least causing an annoyance.
I only want to encourage those out there that are going through a haunting to look at it as an intelligent haunt. It is aware of what it is doing and it is manifesting FOR you.