So, the last look at the three types of Horror brings us to The Terror.
The Terror is simultaneously the easiest and the hardest of the three types of Horror to describe.
(Click the links to see the first two if you haven't read them already.)
The gross-out was the stuff that makes your stomach churn and look away before you hurl your lunch across the ground.
The Horror was the monsters that devour us and evoke our primal fears of dangerous things.
Well, let's start with what The Terror isn't.
The Terror is not the Gross-out. It's not looking to hit your gag reflex and make you spew your lunch across the yard. You're not usually (USUALLY) going to find The Terror in descriptive scenes of copious spouts of blood, splattered viscera, and jellified bones and organs. You may find The Terror alongside such things, but that doesn't make them part of what makes The Terror work.
The Terror is also not The Horror. The Horror lives in the known. It's the giant monster that just wants to eat us. It's the psychopath that, in spite of all his nefarious motives, fetishes, and twisted goals, remains nothing more than human. It's the danger that we're aware of, and yet can't escape with ease or without a fight.
That just emphasizes, once again, that The Horror is based on the unknown. The things we can't understand or even be fully aware of. As Mr. King said, "It's when the lights go out and you feel something behind you, you hear it, you feel its breath against your ear, but when you turn around, there's nothing there..."
I believe The Terror also applies to things in this world that we can't understand, as well. Aliens fall under The Terror, because we have no way of knowing what they're capable of with their technology, and we can have no real understanding of what they want or why they do what they do. Even though they exist in the same world, on the same physical level, they are beyond us so far, and in so many way, that they are all but incomprehensible to us.
Now, I want to finish out by saying while none of these are really inclusive of each other (barring phobias and personal fears which may elevate or demote a subject through the levels), none of them are really exclusive in the boundaries of a good story.
So, rather than these being sub-genre's of Horror, I think we should view these as tools. While you can certainly write an entire book under one of these flags, I think it creates better work if you can incorporate two or even all three levels, much like a good description will hit on multiple senses.
Lastly, I just want to point out a story which, I think, successfully hits on all three types of Horror. As this began with a quote from Stephen King, it will end with him as I wave my hand over this work Children of the Corn. The story hits upon the Gross-Out (The brutal murder of the wife), the Horror (The murderous children), and The Terror (He Who Walks Behind the Rows). Definitely worth the time if you haven't read it already.
Anyway, that does it for this little mini-series. Feel free to comment or contact me if you have anything else to add or want to berate me. This is all my opinion after all.