Possession – state wherein demons take control of the body (not the soul) of the person; they speak and act without the consent of the person and the person later has no knowledge of what transpired while he was under control. This is the greatest form of demonic attack wherein the demons act from within the person. The manifestations of possession can range from an immobile catatonic trance state to a full-fledged, physically violent display of hatred and pride.
Oppression –includes burdens or afflictions that torment from the outside and which are wide-ranging and diverse in nature. This is a lesser form of demonic attack but, if not addressed, can lead to full possession. Oppression can affect one’s health, emotional well-being, spiritual life, relationships and work. These areas of one’s life deteriorate with no apparent cause. With physical and mental deterioration, if it is caused by an evil spirit, medical science would find difficulty in diagnosing the case, much more curing the person of the affliction.
Demonic Obsession– includes compulsions that affect the imagination and the mind which are blasphemous and suicidal in nature. Also included here is the compulsive nature of habitual sin.
Infestation– occurs when extraordinary demonic activity is centered on an animal, thing, or place.
Reference: Catholic Handbook of Deliverance Prayers