What is Horror? Well that can change a bit from person to person so here’s my personal thought on the matter.
Horror is a genre of film that uses both dread and horror as its primary ingredients. Dread being the feeling of anxiety leading up to the act while horror is the feeling of revulsion after the act has occurred. But can be expanded to include any film that wishes to predominately evoke negative emotional responses in its viewers.
This is a very broad definition that can be blanketed over a wide variety of films. So sometimes my personal opinions may clash with the actual classification of film in question.