A nightmare is a bad dream. A night terror might last considerably longer than a nightmare, but, paradoxically, it leaves no memory when the child awakens in the morning. It's not always clear.
Dreams are complex.
Adding to their complexity is the perspective of the Hebrew Bible. It does not treat dreams as possible messages from an adult's psyche to himself or from a child's developing memory to his undeveloped power of reason. Dreams in the Pentateuch are vehicles of prophecy. If by dream we mean a visual experience when asleep, there are only nine dreams in the Pentateuch, and all are in Genesis. Joseph is connected with six of them.