The goal of any horror writer is to terrify the reader as much as possible through words. Those who do not read may not understand the influential power of fiction. Upon first glance, they see nothing more than ink on a page - the black and white. However, when correctly strung together these words can combine setting, characters and events in a way that requires no Hollywood special effects to make the audience's blood run cold.
It is my personal opinion that the imagination can create even more detailed and realistic images than anything a film can produce. Give it the right tools and a nudge in the desired direction and those little black and white letters may as well be thousands of tiny, fanged beasts gnawing away at the reader's mind.
As a writer, it can be difficult to find a good balance between providing information and letting the imagination do the work. If you spell it all out and get too detailed, you may delve into the realm of boring or even cheesy. On the other hand, if you leave it all up to the imagination then you are not really writing anything.
Offer the reader a well placed opening at just the right moment. Seduce them with the ominous setting, and maybe some back story. Tease them with just a glimpse of the entire situation. Then give their imagination a push and let it gain momentum!
While the characters wander dark forests, corridors, streets, or whatever Hell you have placed them in, drag the reader along for the ride. Just as the action creeps around the corner or we are about to view the evil beast, killer, whatever, only allow a glimpse. The description doesn't even have to be visual, it could be something heard, felt or realized.
I believe some miss the importance of imagination. A writer can create excellent fiction with the perfect combination of words and structure. Their grammar and spelling could be flawless. However, without the reader's imagination, it is nothing but words on the page.