Ghost~  Soul, Spirit, Demon. The disembodied spirit of a dead person, conceived of an appearing to the living as a pale, shadowy apparition.

Phantom~ Something that seems to appear to the sight but has no physical exsistance; Apparition, Vision, Spector. Something to be feared or deaded.

Wraith~ Guardian; a ghost. Spectral figure of a person supposedly seen as a premonition just before that person dies.

Demons~  Demons are fallen angels, purely evil and revel in the misfortune of humanity. They thrive off of human suffering. These creatures delight in causing pain and exist solely for wicked purposes. Demons have no sex, meaning they are neither male nor female, yet they may display either male or female anatomy(or a combination of both).
Depending upon which religion you subscribe to, there are varying forms of a demonic heirarcy, with Lucifer being king, and Beelzebub and Astaroth being his highest lieutenants. In extreme cases, demons have been known to possess human beings, and demonic possession is arguably the worst condition which can befall a person.  The only known cure for this ailment is exorcism, a ritual practiced mainly by the Catholic church. It is important to note that there are many different types of factors which could be misinterpreted as demonic possession including drug abuse and mental illness.

Doppelganger~  This is a German term meaning "double." A doppelganger is an exact replica, or double, of a living person who seems to have demonic origins. Very little is known concerning this phenomena and its occurance is vey rare.
 
Exorcism~ Exorcism is the act of driving out demonic infestation by means of ritual prayers and incantations. Most commonly practiced by the Catholic church, exorcism is  frowned upon in today's society and is performed often times in secret. A holy text entitled The Rituale Romanum is typically used while conducting the exorcism, and often times the priest will read the script aloud in Latin. This process can last anywhere from a few hours to several months to complete. During the exorcism, the demon will thrash about, scream profanities, and mock the holy trinity. In extreme cases, the demon will exhibit it's power by levitating objects in the room, talking in foreign languages (sometimes backwards!) unfamiliar to the possessed, and cause scratches to appear on the victim's body. Often times the demon will appear to have exited the victim, only to inexplicably return days later. The priest usually fasts prior to conducting the exorcism and the ritual can be carried on for hours each day, often lasting until late into the night and even the next morning. This is a terribly exhausting process for all involved.  The demon is forced to reveal it's name during the ritual. St. Michael the Archangel is frequently invoked, as he is considered to be the most powerful and adept of all the angels at battling the demonic forces. Finally, the demon is commanded in the name of the Lord to leave the body of the victim.

Incubus and Succubus~  Icubi and Succubi are demons that attack people sexually. Although they are neither male nor female, succubi prey on men and incubi prey on women. Initially the creature will appear as a beautiful person, perhaps the embodiment of what you desire most in the opposite sex. The victim becomes paralyzed and the demon then thrusts itself upon a person, revealing its true nature. They may have contorted, disfigured faces and reptilian or animal-like skin. Reports indicate they are terrifying and utterly horrific to witness. The incidents typically last several minutes, and once finished, the victim is paralyzed until the demon floats away, laughing cruelly all the while. A disgusting thick residue is left covering the hapless victim. This phenomena is also referred to as "old hag" syndrome.

Poltergeist~  The term poltergeist is a German word when translated literally means "noisy ghost." Originally this term was used to describe a haunting in which the ghost seemed mischevious in nature, causing rappings on the wall and moving or hiding objects. The entity seemed to often focus on a child, typically female, in the puberty stage. However, many researchers now believe poltergeist activity to have a much more earthbound origin. Rather than the child being the focus of the phenomena, it is thought the child is actually the source of the phenomena. Children going through puberty are often in a state of emotional turmoil. The theory holds that the child is subconsciously "acting out" through the use of psychic abilities such as telekinesis. When the subject becomes frustrated or anxious, an increase in the poltergeist activity has been noted. Once the child outgrows the puberty stage, the phenomena seems to cease.