American Horror Story co-creator Brad Falchuk had this wonderful phrase to describe the show, saying it's all about "the monsters at home." For example, even though the first season was a ghost story, it was really about the breakdown of a marriage.
Most people correctly refer to horror as a genre of fear. If it's got vampires, monsters, or serial killers hacking away young naked people at a summer camp, it's horror, right? Wrong. I think we need to accept that horror is a little broader than that, more fundamental.
Monsters are a subgenre of horror. Fear is the main point, and even those that we might label more as thrillers are, I think, horror stories. They say that horror is meant to induce terror while thriller is all about the suspense, but it's very difficult - almost impossible - to distinguish between the two.
If you look at the Denzel Washington film Man on Fire, you can't help but wonder about all the terrible things that might be happening to that little girl the longer she's missing. The same thing can be said of the film Taken; the first one, not the insanely unnecessary sequels. You want Liam Neeson's character to find his daughter. You see her friend dead from a forced drug overdose and worry that the daughter has suffered the same fate, maybe even something worse. You can go ahead and add the monsters and the supernatural on top of those stories, but the core premise remains the same.
You can also do the reverse and interpret horror in the more conventional terms of a thriller with the same results. Dracula is basically the story of a serial killer. In fact, Bram Stoker once compared the crimes of Jack the Ripper to those of a vampire.
Fright Night is another great example. I'm referring specifically to the 1985 original when audiences didn't know what to expect. The thing that frightened me about it was that Chris Sarandon's Jerry Dandrige didn't initially come off as a monster. For a while, you think that Charlie Brewster is just letting his imagination get the best of him, because thanks to POV shots, it really could all be in his head. On its surface, Fright Night is a monster story, but the real fear underneath is the notion that something terrifying can infiltrate your neighborhood. It could be a vampire, a serial killer, or an abusive father.
In fact, a lot of times, when someone recommends me a book or a movie that they say is a horror story, I often ask what's the Jerry. What part of the story is simultaneously terrifying and mundane?