A night terror, also known as pavor nocturnus, is a parasomnia sleep disorder characterized by extreme terror and a temporary inability to regain full consciousness. The subject wakes abruptly from the fourth stage of sleep, with waking usually accompanied by gasping, moaning, or screaming. It is often impossible to fully awaken the person, and after the episode the subject normally settles back to sleep without waking. A night terror can occasionally be recalled by the subject. They typically occur during non-REM sleep.

Night terrors versus nightmares
Night terrors are distinct from nightmares in several key ways. First, the subject is not fully awake when roused, and even when efforts are made to awaken the sleeper, he/she may continue to experience the night terror for ten to twenty minutes. Unlike nightmares, night terrors occur during the deepest levels of non-REM sleep. Even if awakened, the subject often cannot remember the episode except for a sense of panic, while nightmares usually can be easily recalled. After the night terror is complete, it can often result in a seizure, which is commonly a tonic-clonic type (although the subject may not be epileptic). The subject often has no recall of the dream, nor the fit.
Unlike nightmares, which are frequently a scary event dream (e.g. a monster under the bed, falling to one's death, etc.), night terrors are not dreams. Usually there is no situation or event (scary or otherwise) that is dreamt, but rather the emotion of fear itself is felt. Often, this is coupled with tension and apprehension without any distinct sounds or visual imagery, although sometimes a vague object of fear is identified by the sufferer. These emotions, generally without a focusing event or scenario, increase emotions in a cumulative effect. The lack of a dream itself leaves those awakened from a night terror in a state of disorientation much more severe than that caused by a normal nightmare. This can include a short period of amnesia during which the subject may be unable to recall his name, location, age, or any other identifying features of himself. This state generally passes after only one or two minutes.
While each night terror is usually different, all episodes of the same person will generally have similar traits. One of the most common qualities of night terrors is a strong sense of danger. For these people there is always a being, tangible or otherwise, who wishes to hurt the sleeping person, or the sleeping person's loved ones. Many sufferers of night terrors are reluctant to speak of them because of their violent and often disturbing nature.


Info Source, Links and more info:
http://en.wikipedia.org/wiki/Sleep_terror_disorder