A dybbuk (pronounced "dih-buk") is the term for a wandering soul that attaches itself to a living person and controls that person's behavior to accomplish a task. The word "dybbuk" is the Hebrew word for "cleaving" or "clinging," and having a dybbuk is not always a bad thing for the human host...though, in most instances this is not the case.
In the Roman Catholic view, a person can succumb to a demon or devil that takes over their body, and the only cure is an exorcism to drive the demon out. In the Jewish faith there is no belief in demonic possession though there can be a possession of a living person by the soul of one who has left the body, but not the world. This soul is seeking a body to possess in order to take care of unfinished business.
In the Old Testament of the Bible, a bad spirit is described as attaching itself to King Saul, the first chieftain of the ancient tribes of Israel. Later, the prophet Elijah is possessed by the spirit of a dead man in an attempt to persuade the prophet into maneuvering the King into a war.
According to ancient Hebrew tradition, demons are beings not much different than humans but were created in the twilight of creation after the humans and right before the end of creation. They are neither of our world, nor of the other world, but a part of both.
The concept of the transmigration of souls developed and found serious followers during the Dark Ages, and by the 12th century it became an established part of the Kabbalah. The 16th century schools of mysticism embraced it. When Hasidism developed, the belief took final hold.
The first form is the Gilgul, which is the Hebrew word for 'rolling,' but means, in this context, the transmigration of the soul. Generally, it is represented as a natural sequence in the life of the soul and simply enters the body at birth, just as the infant is about to leave the mother's body, and prepares to live whatever normal life span has been allotted to it.
The second form of transmigration is the Dybbuk, a disembodied spirit possessing a living body that belongs to another soul. The earliest description was that they may be nonhuman demons...later it was assumed they were the spirits of persons who have died. The dybbuk may be the soul of a sinner, who wishes to escape the just punishment given to it by the 'angels of the grave' who seek to beat them, or to avoid another form of soul punishment...which is wandering the earth. A dybbuk may seek revenge for some evil that was done to it while it lived. The dybbuk may be lost and enter a body simply in order to find a rabbi who would be able send it on it's way. The living host may or may not know that a dybbuk is occupying their body. There may be torment towards the host but this depends on the intent of the possessing soul.
The third form is the Ibbur. The Hebrew translation of the word means 'impregnation.' Ibbur is the most positive form of possession, and probably the most complicated. It happens when a righteous soul decides to occupy a living person's body for a time, and joins, or spiritually 'impregnates' the existing soul. Ibbur is always temporary, and the living person may or may not know that it has taken place. Many times the living person has given consent for the Ibbur since it is always benevolent. The departed soul seeks to complete an important task, to fulfill a promise, or to perform a Mitzva (a religious duty) that can only be accomplished in the flesh.
If the dybbuk is able to encounter a rabbi while possessing a living host, then an exorcism ritual can be performed. The Jewish exorcism ritual is performed by a rabbi who has mastered practical Kabbalah. The point of the exorcism is to heal the person being possessed and the spirit doing the possessing. This is a stark contrast to the Roman Catholic exorcism that is intended to drive away the offending spirit or demon. The intent is to heal the soul that's possessing and heal the person. The ceremony is done on behalf of both.
In some cases, there is a positive aspect to a dybbuk. On occassion a spirit will seek out a person in a time of need to help. This second type of possession is called 'sod ha'ibbur,' which is Hebrew for 'mystery impregnation.' This is a good possession simply because this is a spirit guide. The spirit of someone who has struggled and overcome what you have struggled with and can't overcome will be lent to you from the spirit world to possess and stimulate you, and help you overcome misfortune and what it has been able to in its lifetime. When it's tasks are done and you've managed to achieve what you need in your life, it leaves you.
Some people reach high pinnacles of achievement and they may fall into deep depression. This can be explained as the loss of that spirit. There's a sense of loss, and it's misinterpreted as depression. If the person eventually realizes that, they can be thankful that they had a spirit guide to help them. They need to continue to lift up their own spirit for the remainder of their life.
The concept of dybbuk recognizes that our physical world and the spiritual world can intertwine for both positive and negative reasons. If those intersecting reasons are negative, there is a healing process to mend the collision so both the possessor and the possessed can move on.
NOTE: There have also been times when a dybbuk can haunt or infest an inanimate object, for example the Dibbuk Box, a supposed haunted wine box that has brought terror and regret to several owners over the years...Lon
Sources:
www.ghostvillage.com
www.metro.co.uk
www.britannica.com
www.myjewishlearning.com
www.mythencyclopedia.com
learnkabbalah.com