What is Horror?

Horror and our fascination with it go back as far as human memory stretches.  Ghosts and devils and all manner of evil deeds are fundamental elements in our story telling traditions. While this genre is not respected in main stream fiction or in mass media it has a power that transcends critics and fads.
What horrifies us is a deeply personal affair. The story that chills one person to the bone leaves another bland and unmoved. We all have our different psychological triggers and horror works best when it presses on one of the triggers in an unexpected fashion.  Horror is much like pornography; tough to define we know it when we see it. Despite the individualistic nature of our responses there are some comment elements to horror and in these we might find an answer to what is horror really?

On one basic level horror is when the world no longer conform to the rules we have accepted. There’s a definition of horror that goes when there is a knock at your door at 1 am and you open to door to find a clown, which is horror. The clown is unexpected; it violates the rules for who would be at your door at that absurd hour of the morning. The supernatural is invoked as horror because it is supernatural, it is a violation of all the rules we’ve been taught about the world. The dead stay in the graves, people do not assume animal shapes, and there are no malignant intelligences bartering for our souls. Any of these events happening would be horrifying because it would be such a divergence from reality as we know it. In fiction all of these have been seen and explored so many times that it is difficult to make to horrific as we have an understanding of the kind of rules they might obey. The talent horror author finds a way to explore new rule-sets unknown to the audience that brings horror back into the equation.
Too often what someone intends as horror turns into comedy and farce. This is because comedy also relies upon the unexpected and that which breaks rules. The punch line of a joke is the moment when the unexpected is delivered, and like horror if the result is seen before it is revealed it has no power. The difficult trick in writing horror and not turning it into farce is to make the unexpected believable. It should be unexpected before the horror but seem acceptable and inevitable after the reveal.
Shock and gore are used by many to try to achieve horror but these techniques primarily generate startle and surprise. These elements can contribute to horror but they are unlikely to achieve it on their own. Revulsion isn’t horror and the shock of being startled fades quickly and leave no lasting impression. To me horror is that which lingers after the effect has been generated. It is the inability to keep one’s thoughts away from the unsettling revelation the generated the horror.
Another thing used all too often as a short cut to horror and nearly always fails is to cheat the audience or readers by withholding vital information until the moment of the reveal. This is as poor writing horror as it is in mystery. The best horror lays out the vital information but in such a way it is discounted or the audience assumes it to be something other than it is. You can mislead the audience and this can be an effective technique in horror, but you can’t get away with lying to them.
My personal metaphor for horror is the moment the ground slips under your feet. For that fraction of second when the ground first gives way before your rational mind understands that you are falling you are in a state of horror. If that moment is capture and extended then you have a real emotion of horror. The world is no longer what you though it was. What was once solid and predictable is now unknown and dangerous. With falling the moment passes quickly, but in a good horror story the audience is left unsettled never regaining their balance. This is achieved quite rarely, but when it is it is unforgettable.
You don’t need the supernatural to do this, though it makes it easier. Gregory Benford acheived this in his short story A Dance To Strange Musicsa horror story that is also hard-SF.