There is spirit possession and demonic possession, and different kinds of each.
People who are prone to spirit possession, that is, in cultures in which spirit possession is accepted as real (the Hofriyati of Northern Sudan; the Tongans in the South Pacific). In these cultures, spirit possession is usually a part of healing: physical, emotional, mental or cultural. The type of possession depends on the type of spirit. For example, in Samoa, a person can be possessed by either an ancestral spirit or by the spirit of a former god or goddess from the pre-Christian religion. Ancesral spirits tend to possess a person in order to address wrongs committed in the family or community, usually having to do with hierarchical respect, and often against the person who is possessed. God spirits usually possess a person who has committed some act of sexual disrespect, such as showing a thigh or wearing one's hair down. The spirit leaves the person once these wrongs have been righted and the guilty parties have been punished. The act of righting wrongs can be healing on many levels. A spirit can also "hit" a person and cause illness, but this is not possession in the tradtional sense; more like obsession (as defined above). In some cultures, spirits are propitiated to encourage them to leave; in others, they are forcibly removed. Both methods can be utilized, depending on the situation. In Samoa, for example, if a spirit "hits" a person and makes them ill, the native healer will deal cards to determin which spirits (usually spirits of the deceased rather than god spirits) are causing the trouble and why, and they are forcibly removed.
In demonic possession, it is an "unclean spirit," demon or devil that is possessing the person. Malachi Martin said that a person has to invite the demon in before possession can take place. Demons can be very sneaky in order to gain an invitation.
The chief things formally exorcised in blessing are water, salt, oil, and these in turn are used in personal exorcisms, and in blessing or consecrating places (e.g. churches) and objects (e.g. altars, sacred vessels, church bells) connected with public worship, or intended for private devotion. Holy water, the sacramental with which the ordinary faithful are most familiar, is a mixture of exorcised water and exorcised salt; and in the prayer of blessing, God is besought to endow these material elements with a supernatural power of protecting those who use them with faith against all the attacks of the devil. This kind of indirect exorcism by means of exorcised objects is an extension of the original idea; but it introduces no new principle, and it has been used in the Church from the earliest ages. (Catholic Encyclopedia)
In Catholic exorcisms, the Roman Ritual of Exorcism is used.
In ... authentic cases of possession ... every priest, especially if he be a parish priest, or pastor, is liable to be called upon to perform his duty as exorcist. In doing so, he is to be mindful of the prescriptions of the Roman Ritual and of the laws of provincial or diocesan synods, which for most part require that the bishop should be consulted and his authorization obtained before exorcism is attempted. The chief points of importance in the instructions of the Roman Ritual, prefixed to the rite itself, are as follows:
- Possession is not lightly to be taken for granted. Each case is to be carefully examined and great caution to be used in distinguishing genuine possession from certain forms of disease.
- The priest who undertakes the office should be himself a holy man, of a blameless life, intelligent, courageous, humble, and he should prepare for the work by special acts of devotion and mortification, particularly by prayer and a fasting (Matthew 17:20).
- He should avoid in the a course of the rite everything that savours of superstition, and should leave the medical aspects of the case to qualified physicians.
- He should admonish the possessed, in so far as the latter is capable, to dispose himself for the exorcism by prayer, fasting, confession, and communion, and while the rite is in progress to excite within himself a lively faith in God's goodness, and a patient resignation to His holy will.
- The exorcism should take place in the Church or some other sacred place, if convenient; but if on account of sickness or for other legitimate reasons, it takes place in a private house, witnesses (preferably members of the family) should be present: this is specially enjoined, as a measure of precaution, in case the subject is a woman.
- All idle and curious questioning of the demon should be avoided, and the prayers and aspirations should be read with great faith, humility, and fervour, and with a consciousness of power and authority.
- The Blessed Sacrament is not to be brought near the body of the obsessed during exorcism for fear of possible irreverence; but the crucifix, holy water, and, where available, relics of the saints are to be employed.
- If expulsion of the evil spirit is not obtained at once, the rite should be repeated, if need be, several times.
- The exorcist should be vested in surplice, and violet stole.
During the Roman RItual of Exorcism, the priest says several prayers and those present respond at the appropriate times. Then he commands the demon as follows:
I command you, unclean spirit, whoever you are, along with all
your minions now attacking this servant of God, by the mysteries
of the incarnation, passion, resurrection, and ascension of our
Lord Jesus Christ, by the descent of the Holy Spirit, by the
coming of our Lord for judgment, that you tell me by some sign
your name, and the day and hour of your departure. I command you,
moveover, to obey me to the letter, I who am a minister of God
despite my unworthiness; nor shall you be emboldened to harm in
any way this creature of God, or the bystanders, or any of their
The ritual is much longer. If you are interested, you can read the entire 1964 ritual at the link above.
Dissociation in psychological circles is also called MPD or Multiple Personality Disorder. The longer name of dissociation is Dissociative Identity Disorder.
Many researchers think that a possessed person is experiencing a temporary dissociation, rather than a large-scale identity disorder.
"Pretty much everyone dissociates at some time or another, and dissociation is a normal childhood developmental cognitive process. This is a natural, normal, necessary process which is built into the human psyche. In the mildest forms, dissociation includes the way people can "zone out" when they are bored or totally lose themselves in a daydream or a book or a movie, or concentrate on a project so fully that they lose track of time or their own body signals (such as hunger).
Dissociation is a form of selective attention and imagination. Without those capabilities, one would never be able to learn, make connections in thought, or develop intuition in relationships. - Dr. Dan B. Allender
In the most severe form, chronic dissociation may take the form of poly-fragmented Dissociative Identity Disorder, which, until recently, was known as Multiple Personality Disorder (and is in no way related to nor is it the same as schitzophrenia, which is a specific brain illness and is chemical and biological in nature). There is, in fact, quite a broad spectrum of dissociation, and that's even a "sliding" scale, as some people experience more or less dissociation from day to day depending on any number of factors (stress seems to be a big one)."
Many Christians believe that dissociative disorder and demonic possession are two distinct events and must be distinguished before an exorcism is attempted. (see an article in Christianity Today)