Difference. As george Watsky says, "The difference is in the differences."

You have nightmares - those dreams that reach out and grip you in fear...in terror. The average nightmare is our inability to face our darker sides. It's our fears personified, amplified, and brought to life.

Then you have night terrors. These are recourring, CHRONIC, nightmares. There is almost always a chemical or mental imbalance at the source of these, and typically the only way to avoid them is dreamer empowerment and/or medication. Regardless, these still exube the same inner shadows that drive the average nightmare. They recur because our imbal;ances prevent us form overcoming the monster head on. What was amplified is now obsessively resurrected every night. It's a battle, but it can be one...or at least survived.

Then there is...the non-human dreams. Those dreams where we must tread the difinition very cautiously, not allowing ourselves to give credit where it isn't due. Angels, demons, spirits, and any other beings you might choose to (or to not) believe in can access the dreaming realm. Religions and cultures across the globe, from eras back onward, give note to the dream as a place where inspiration, revelation, epiphany, and in some cases assault of the spirit can occur. But we must make sure every plausible alternitive is dis-proven before we make the statement that someone has been visited by any sort of being. Remember - hell and heaven are almost never (and by that I mean more than 99% of the time) actually the places depicted.

The best defense against nightmares is self empowerment. Study up on lucid dreaming and dream visualization. Seek counselling for the traumas that haunt you (for only when we grow in peace can we truly control the every aspect of our dream). Love yourself; even in non-human dreams, most of what you see relates to you the dreamer.