Over the past century, horror as a distinct genre has grown quite a bit and contains numerous subcategories, with various levels of the macabre, gore, and realism tossed in. All things aside, when it comes to using horror in gaming, whatever the subgenre, it all boils down to being able to instill real terror into the PCs. You want to make neck hairs bristle, you want them so paranoid they begin to distrust each other, you want them in a state of hyper-alertness and anticipation because in those states, the characters (and sometimes even their players) begin to place so little efforts on the now, and so much of their energy futures that they lose site of the presence. The future, since it cannot actually be fully actualized become the plaything of the imagination— and if the dial for imagination is set to fear, the players will imagine far more horrible things to happening them than you ever could. Why? Because each of them possesses their own set of fears, each have their own nightmares, and your players will voice them in anticipation. That's when you as GM should take notes, and write them down… you’ll need them later.