Terror is usually described as the feeling of dread and anticipation that precedes the horrifying experience. By contrast, horror is the feeling of revulsion that usually follows a frightening sight, sound, or otherwise experience.
The terror is experienced in the development of suspense and the fear of the unknown. The horror is manifested through the unknown becoming the 'known', and the fear being realised. The conventions of horror films delve into our fears and bring to the surface the degeneracy of the human mind.
