Monsters are a staple of horror fiction. Stories can be built around classic monsters, such as vampires, werewolves or ghosts. These stories are usually built around the idea of the monster of choice terrorizing humanity. The hero of the story, often the only one who understands the true nature of the danger, is tasked with defeating the monster.
Evil is a word that is not easily defined, but when we speak of human evil in horror fiction, we generally mean something like a serial killer being on the loose or someone going crazy and committing horrible acts. Horror fiction of this kind can be very disturbing because it is largely based in reality and readers can easily believe the premise.
Horror stories often involve a mix of science fiction. These stories became increasingly popular in the 1950s and '60s as humanity was making great scientific advancements and many people could easily imagine the science spinning out of control. Horror stories of this type usually rely on the idea that some scientific experiment has gone wrong, perhaps creating a monster, causing some catastrophic plague or inviting extraterrestrial beings into our lives. Horror fiction of this type works because we can recognize the possibilities of the horror happening, even though it may not be a reality at present.
Emotional horror is a subtle sort of horror story idea. These stories usually deal with a perceived fear or terror, rather than something tangible. It may be a fear of losing someone or a fear of the dark or a fear of bugs. These fears are then magnified in the mind or life of the protagonist who then has to deal with them or be consumed by them.