Theories on ghostly activity abound! Below I've listed a few of the more common ones:
Ghosts are residual energy of a person or animal that has been "recorded" in the environment.
Ghosts are hallucinations that are caused by high electromagnetic field fluctuations or low frequency soundwaves.
Ghosts are spiritual entities that have never been human (demons).
Ghosts are a manifestation of human consciousness operating after death.
Ghosts are recurrent spontaneous psychokinesis (RSPK)--this pertains to poltergeists. Poltergeist activity is thought to be centered around and the result of a living person unconsciously relieving stress by mentally moving objects, causing noises, etc.
For more information on ghostly theories, I suggest you check out the Association for the Scientific Study of Anomalous Phenomena's (ASSAP) page on this:
Types of Hauntings
Residual hauntings are thought to be like a video playback of a past event. The "ghosts" don't interact with the living in any way. The environment simply "plays" the same scene over and over. Some individuals believe that if a person has performed a repetitious act for a long time, he or she will have left a psychic impression or "groove" in that area (I'll be seen long after my death feeding my dogs--I just know it). These "ghosts" appear when the atmospheric conditions are right. It is like switching on your DVD player and watching a movie. Personally, I believe it is this type of phenomenon that has the best chance of being accepted into the scientific mainstream as it affords the opportunity to be repeatedly studied at length and properly documented--once researchers figure out how to turn on the DVD player!
Interactive hauntings are a different story. These hauntings are just like the name says--interactive. There is actually an intelligence behind this phenomenon that can and does interact with the living. Just what is that intelligence? Once again, no one knows for sure. Researchers at the Rhine Research Center believe that these intelligent entities are human consciousness operating after death and that people perceive them through psi abilities such as telepathy. After all, ghosts don't have physical hands, feet, mouths, etc. Therefore, people "see" them and interact with them with their mind's eye. This theory explains why some people in a room can see or hear a ghost and others can't. For more information on both of these theories, I suggest you check out the Rhine Research Center:
The main research topic I'm focused on is the interaction between people and ghostly episodes in reportedly haunted places. There seems to be an interaction between the environment and the people in it which can lead to paranormal activity. I'm also interested in correlations between magnetic fields and anomalous activity as there seems to be a connection there as well. Finally, I'm hoping to some day do a case study of a poltergeist (top on my wish list!). There are a lot of great research studies out there on ghostly activity. Here are a few:
A Rasch Hierarchy of Haunt and Poltergeist Experiences by Houran and Lange
Poltergeists, Electromagnetism and Consciousness by Roll
A Life Less Orbinary? Accounts of Experimentation into the Natural Causes of 'Orbs' by Wood
In Search of Magnetic Anomalies Associated with Haunt-Type Experiences: Pulses and Patterns in Dual Time Synchronized Measurements by Braithwaite, Perez-Aquino and Townsend
Using Digital Magnetometry to Quantify Anomalous Magnetic Fields Associated with Spontaneous Strange Experiences: The Magnetic Anomaly Detection System (MADS) by Braithwaite