When the Church asks publicly and authoritatively in the name of Jesus Christ that a person or object be protected against the power of the Evil One and withdrawn from his dominion, it is called *exorcism*. Jesus performed exorcisms and from him the Church has received the power and office of exorcizing. In a simple form, exorcism is performed at the celebration of Baptism. The solemn exorcism, called a "major exorcism," can be performed only by a priest and with the permission of the bishop. The priest must proceed with prudence, strictly observing the rules established by the Church. Exorcism is directed at the expulsion of demons or to the liberation from demonic possession through the spiritual authority which Jesus entrusted to his Church. Illness, especially psychological illness, is a very different matter; treating this is the concern of medical science. Therefore, before an exorcism is performed, it is important to ascertain that one is dealing with the presence of the Evil One, and not an illness.If you're wondering about the minor exorcism at baptism, BTW, I attended a baptism this past weekend; it basically consists of a short prayer to cleanse the person about to be baptized from original sin. The baptismal promises, a statement of faith made either by the candidate or the parents/godparents if the candidate is an infant, also start with an explicit renunciation of Satan. The minor exorcism at baptism does not presume that the candidate is actually under demonic possession, but is part of the overall process of baptism, in which the baptized one 'dies' to all sin and is 'reborn' in Christ.
(Catechism of the Catholic Church, paragraph 1673.)
 Cf. Mark 1:25-26; 3:15; 6:7,13; 16:17.
 Cf. CIC, can. 1172 [CIC is the Code of Canon Law -- John Ockerbloom]
Demonic possession in Catholic belief involves malevolent angels rather than deluded aliens, and is treated as an extraordinary case (for which other explanations like mental illness must be considered first) rather than ordinary (how many BTs is the average person supposed to have?) Exorcism must be done specifically in the name of Jesus by a priest with permission from the bishop and following certain careful rules, rather than by oneself using an e-meter and no divine invocation. You also don't have to go through years of indoctrination, or pay thousands of dollars, to get exorcised.