Night terrors is when children wake in the middle of the night and scream or cry but the parent has no idea why.  It seems as if the child is still asleep but he can be sitting up in bed with his eyes open. Yet, in the morning, the child doesn't remember the incident at all.  Night terrors are different than nightmares in that they occur at night instead of early morning and the child has no memory of it.  This article explains in detail what night terrors are and gives tips to parents on how to handle this when it happens.