According to Webster's dictionary the word "haunting" is defined as: 1: to visit often, or to continually seek the company of   2: to have a disquieting or harmful effect on, or to linger.   From this definition we can see what a ghost 'does' but it does not help tell us what it is.

We can therefore determine that a "haunting" is simply repeated unwanted visits that disturb or harm.   Visits from who or what though???

The term "spirit" is defined as a supernatural being.   Although I believe that there is nothing "SUPER" natural about spirits.   They are simply natural occurrences that surround us every day, everywhere.   For the great majority of us we function completely oblivious to their existence, this creates for us a "comfort zone" about ghosts.   This comfort zone is disturbed or completely undone when we are unexpectedly required to meet and deal with these spirit forms.
Nothing on earth will convert a skeptic quicker than having a ghostly encounter.
All religious philosophies aside, the "essence" that makes us human is the "energy" that some call a soul.   One of the basic concepts in science is that "energy" cannot be destroyed, it only changes form.   With that fact in mind we will explore the realm of spirits.
I would like to say here that I cannot present to you all that my studies and experiences have taught me, only that I will try to cover the highlights and most frequently asked questions.   Also, I do not have all the answers, but am willing to work with you as an individual to find whatever information that will help.   Further, I will not enter into a religious debates or philosophical discussions.   It has been my experience that ones religious foundation will color attitudes and beliefs until the day you die, rather I will stick to practical, helpful activities that will guide you through your encounters.

Why do Haunting's occur?
Haunting's occur for one reason, that the essence/spirit of a human is bound to this plane.
What causes a spirit to remain earth bound? This question is a rather complex one, and the answer varies according to the spirits personality in life.   Some reasons that are commonly accepted are:  
* Unfinished Business, the spirit feels that he/she has something important to accomplish before moving on.     
* Attachment to another person, the spirit is unwilling to leave this plane of existence because of his/her love for another.
* Material Attachment, the spirit is unwilling to leave earthly possessions.
* Protector, the spirit feels it is necessary to remain as a guardian over another.
* Confusion, the spirit may have had an unexpected traumatic death and is unaware that he/she is now on the other side. 

Although there may be be some other individual reason the spirit remains, most can be classified into one of these categories.
Why me, why my house? Don't take it personally!   In about 99.9% of the cases of haunting's, it is a spirit that you do not know, for reasons you will most likely never figure out.
Are these haunting's random? There is no solid answer to that question. Some haunting's may be based on the actual site, in other words the spirit may have an attachment to the spot your house sits on.   In a few cases though people have done something to "invite" a ghost in.  
Can I get rid of them?
Truthfully, that depends.   There are no sure fire methods or rites that will guarantee you can remove unwanted entities from your home.   Keeping in mind that entities have individual personalities - just like their human counterparts, some may not want to go, regardless of how persuasive and/or forceful you may be.   They may go, or again they may return.   It also depends very much on the "type" of spirit you are dealing with. (please see "types of spirits for more detail)  This not said to discourage you, I have found for the most part that they will leave upon request and no special ceremony is required.