What is an Exorcism?:
The English word exorcism comes from the Greek exorkosis, which means 'out-oath.' An exorcism is an attempt to evict demons or spirits from the body of a (normally living) human being. Belief that demons can be evicted follows automatically from belief in demons capable of possessing people. So long as there is a belief that demons can posses a person, there will be belief that some people have power over those demons, forcing them to cease their possession.
Exorcism & Religion:
The exorcism of evil spirits is commonly associated with the ministry of Jesus. They appear somewhat frequently in the New Testament, but this is curious given the fact that they are absent in the Hebrew scriptures. It appears that belief in demons and exorcism only became really popular in 1st century Judaism, with the Pharisees actively engaged in identifying and evicting demons from people.
Exorcism & Mental Illness:
Many stories of exorcisms appear to involve people who suffer from mental illnesses - understandable, given the fact that medical understanding of mental illness is a relatively recent development. Unfortunately, if a mentally ill persons assumes the traditional symptoms of demon possession, attempts to perform an exorcism will only feed their delusions and keep them from getting real help with a counselor.