What makes a horror film?

I started to research the general conventions of a horror film and what goes into it so that it is actually classed as a "horror"
Wikipedia has it's own definition of the horror genre which is ;
"Horror films are movies that strive to elicit the emotions of fear, horror and terror from viewers. Their plots frequently involve themes of death, the supernatural or mental illness. Many horror movies also include a central villain."

Further reading has revealed that there are 3 types of sub genres horror's which are usually the main underlying plot for these types of film.

There are films which are "the horror-of personality" for example Texas chain saw massacre , where the main villain is mentally instable. The second sub genre is "the horror-of-armageddon"

which are things such as war films. The third and final sub genre is " horror-of-the-demonic" which are films such as the exorcism of emily rose / the exorcist.