Night terrors are very different from nightmares in that night terrors occur during non-REM sleep; nightmare episodes occur during REM sleep.
In addition to occurring at different times of sleep, night terrors involve frightened expressions, bizarre calls, and extremely obvious autonomic symptoms that can be alarming to bystanders, but are not recalled or feared by the person afterwards. In night terrors, the person awakens from a nightmare, is frightened, and can clearly recall it afterwards. It is difficult to be awakened in night terrors, and there are usually no dreams, whereas the opposite is true of nightmares.