Based on the title of this website "Ghosts! Are they real?", you'll know that it's about the reality of ghosts, and haunting's. To be perfectly honest, nobody knows, with absolute certainty whether entities such as ghosts are real or not, but if we make the assumption for a few moments that ghosts are real. Then in my opinion the next logical question to ask is. What are they? What are ghosts? The simple answer to this question is that, ghosts are the disembodied spirits or souls of once living persons. This definition, is in my opinion, not very satisfying to anyone, believer or skeptic. Most persons in the field of the paranormal believe there are two types of haunting's and ghosts.
The first type of ghost would be that of a residual ghost. It is a type of ghost, that is not real in any empirical or physical manner. I would not refer to this type of ghost as a spirit of a once living person, because the essence or energy that animated that person in life would not be present. It would not have any intelligence, personality or physical mass and it would not be able to manifest itself at will, nor would it be able to interact with the living in any meaningful way.
A ghost of this type would very much be like watching a motion picture, that has an actor in it, who is now deceased. The only real difference, it would be more akin to a 3 dimensional holographic image of that actor. It, the ghost would be like an image of that once living person that somehow imprinted itself upon the physical environment.
I am not a physicist and my knowledge of quantum mechanics or quantum physics can be summed up in basically two words, not much! However, I do believe that this would probably occur as the result of the photons of light that were once reflected off that person. And somehow, these photons interacted with the matter in the immediate vicinity on a subatomic or quantum level. And this interaction of the photons and matter may cause an alteration of the electromagnetic structure of that matter, much like sounds or images can alter the magnetic structure of the ferric oxide that's used on magnetic tape.
I also suppose that given the right set of circumstances (time of day, temperature, etc.) the physical matter within that area may be able to project a 3 dimensional image of that once living person. And this image may appear as an apparition or ghost, and it would probably be able to be played over and over again, much like a movie or tape recording. And this may also explain why ghosts of this type can appear to walk through walls, because there is no physical matter, it is simply a projection of an image that appears 3 dimensional. I think this may also help explain why some paranormal activity may increases when a supposedly haunted house or building is being remodeled; the physical items within the building that held this energy are being moved and removed. This disruption, may ultimately cause the activity to suddenly increase or perhaps even decrease or possibly even stop, depending upon the amount of remodeling that has been done and whether any of the original materials were reused.
The second type of ghost would be that of an intelligent spirit. These ghosts, if real, can and sometimes do interact with the living. An intelligent spirit is most likely the disembodied spirit or soul of a once living being, probably human, but I believe could also be animal.
Many paranormal researchers also believe that ghosts of this type are probably made of electromagnetic energy. The problem with this idea, at least the way I understand physics is that just about everything in our universe is comprised of electromagnetic energy of one form or another, from gamma rays, x-rays, ultraviolet, visible light waves and all the other forms of light radiation. And basically all matter within our universe on a subatomic or quantum level is electromagnet in nature, like the electrons, and all the particles that make up the protons and neutrons. If you're an adherent to or believer in string theory, then all matter is nothing more than strings of electromagnetic energy that vibrates at specific resonances to create what we perceive as the elementary particles, different resonances create different particles.
I don't really know what ghosts are but if they do exist then I have to believe they are more likely a disturbance of the electromagnetic field in a supposedly haunted location or perhaps even of the earth itself. How ghosts could disrupt or disturb the electromagnetic field I have no idea, it is only my speculation. But this I think could help explain why some instrumentation like the EMF (electromagnetic field) meters may react to potential paranormal activity. This may also be the reason for cold spots, as heat itself is also a form of electromagnetic energy, and a distortion in that energy could conceivably cause either a cold or perhaps even a warm or hot spot. Please understand that these ideas a purely speculative on my part and do not offer proof for or against the existence of ghosts or the paranormal.
If ghosts are real, then I believe that these ghosts or spirits, or whatever you want to call them may hang around in our reality for a while. How long they hang around in our reality I think depends on what that individual or person believed before their death, and what he or she may expect after death.
If they believed in Heaven and Hell as most Christian religions do, then perhaps that spirit goes to where it believes it belongs. If they believed in reincarnation as some eastern religions do, then perhaps that spirit or soul migrates to an unborn fetus and is then reborn or reincarnated as another person, with no memory of his or her previous life. But on the other hand if that person did not have any solid belief system, then perhaps they continue to exist in our reality for quite a while. And I suppose it is also possible that some may continue in our reality to haunt a particular person or place. I don't use the word haunt in any negative way. It may be just that the person does not realize they've passed on, has unfinished business or wishes to communicate with, or protect the loved ones who may have survived them. Or the ghost of that person may even have a desire to scare someone they disliked. I think it may also possible some ghosts may be trapped in our reality, and are not able to leave as some form of punishment, as an example, the ghost of a prisoner who may haunt the prison where he was once incarcerated.
As to whether evil spirits exist or not, again my answer is, I don't know! But I do believe it possible that when an evil person dies then his or her evil spirit may continue and by the same token when a good person dies than a good spirit may continue. I have a tendency not to believe in inhuman demons, devils or anything of that nature. I believe these ideas were created by religious leaders throughout the centuries to help keep their followers in line, and are nothing more than pure bunk, my personal opinion only.