Night terrors usually happen within a few hours after a child has fallen asleep and can last anywhere from a few minutes to a half-hour. The child generally falls back to sleep and wakes up the next morning without remembering anything about it. On the other hand, nightmares happen much later into the night when the child is dreaming. A child who is having a nightmare tends to have little interest in going back to sleep and needs comforting. You may even need to stay in the room for a while, but at least she can be consoled.
Night terrors happen to a small percentage of children, while nightmares are upsetting dreams that happen to all kids. As children get older, night terrors happen less often. No special treatment is usually necessary. Don't try to wake her up, but do keep toys and any sharp edges that she might hit while she is thrashing around out of harm's way. Be patient and ask her pediatrician for more information and reassurance.