Conventions of a Horror....

Horror conventions tend to create a lot of enigma, this is one of the most important things to have in a horror movie with out it the suspense drops dramatically and therefore isn't as scary, so the viewer isn't kept as hooked as they could be.
Mise-en-scene...
Costume:- victims tend to have causual clothing, murderers wearing black or dark clothing, stained with blood? Ghosts or paranormal subjects tend to be foggy, not scene or done through editing/special effects.
Lighting:- it is usually low key lighting and normally takes place at night.
Actors:- there is normally a 'bad' guy and a 'vunerable' woman.
Make-up:- usually fake blood used or masks to cover the 'bad' guys face.
Props:- there tends to be some sort of murder weapon or an object which connotes scariness.
Setting:- locations are spooky, such as a woods or haunted mansion.
Editing...
The editing is normally fast and pacy which adds to the suspense and jumpiness of the horror film.
Sound...
There are lots of sounds related to horror movies, such as deep breathing, screaming or heavy footsteps. Or if set in a woods the snapping of branches.
Camera...
Lots of handheld shots to add to the mood of the horror.