The key conventions of horror

Don't
Ignore
Setting
Technical code
Iconography
Narritive structure
Character type
Themes

These are the six genre conventions which catergorise what films belong to what genre.

Setting
Usually in horrors the location will be a small community or isolated place. This allows for the viewers to feel a sense of isolation and therefore create a higher sense of fear. If the setting was an urban place with lots of people there would be less isolation and therefore I think people wouldn't feel a sense of horror.
Being more specific the setting of horror scenes are usually places like haunted houses (sometimes of an old fashioned style), churches and forests or deserted rural areas. These places which have a long history or have been around for very long times usually carry a 'past' which could be the source of horror. Furthermore, these places will have somewhere for the villain or horror figure to hide and jump out on the victims. This sense of the unknown creates a lot of fright for the viewers. These types of places are always dark as well which makes it hard for victims and viewers to see which again creates a scary scenario as they don't know where the villain could appear from. Here is an example of how an isolated setting can add to the horror..
Technical code
Generally the camera work is used to create a disorientated view in horrors, this creates unfamiliarity for the viewers which brings them out of their comfort zone. The camera work is expressive rather than naturalistic with lots of high and low angles being used. In horrors, there are many extreme close ups on the victims which shows the severity of the fright they are experiencing. This then allows for the viwers to understand the type of horror scenario the victim is in.
Furthermore, in some horrors hand held cameras are used to create a sense of realism. This in turn makes the horror seem more realistic. For example in paranormal activity the use of a hand held camera allows us to be 'in the room' with the person holding the camera and seems to make it more personal to us.
In some horrors, editing can create jumpy moments for the audience. Editing may cut from a long shot to a close up to create a sense of disorientation. Editing can also be used to make suspense as speeding up the editing pace during scenes creates jumpiness.

Iconography
In pretty much all horror scenes colours of red and black are used as these are seen as colours associated with horror. Red represents blood which means there has a been conflict or a killing whereas black is used as it creates a sense of the unknown as it makes it hard for villains to be seen.
When it is dark villains seem to appear as shadows which creates fear for the victim when they see an intimidating shadow. The shadow also only shows the outline of the villain and there is a sense of the unknown as we cannot see anything else apart from the figure.
Also, there are a lot of objects that we can expect to see in horrors which are connotated with scary films, for example, crucifixes, blood, masks, weapons, ghosts and many more. When we see these objects we can expect horror and this will create a sense of suspense.

Narritive Structure
Horrors are usually in a classic realist/classic hollywood structure. Sometimes horrors will follow the normal path where the 'final girl' who survives will be a virginal and slightly masculine. This seems to suggest that the 'stronger' characters will survive. Most of the time, there will be a clear hero who will eventually slay the villain or save the local town or community.
There are many different theories in terioes in terms of structure. Some horrors will follow the traditional structure of Todorov whereas some will use Barthe's and Levi Strauss' which use mythic structures which create a sense of unfamiliarity.

Character types
In most traditional horrors there will be one main villain and one main hero with a number of victims. The hero will usually be the masculine strong male who leads the group of victims to safety but will usually have to make sacrifices along the way. Villains or monsters will have an unknown motivation for doing what they are doing and will usually be psychotic by an earlier event.
In many horrors the first people to die will be the stupid, immoral teenagers who don't take the situation seriosuly and therefore face the consequences.
In more recent horrors there appears to be more and more occasions where a young, vulnerable child will be a villain and be the cause of death for a lot of people. This goes against the normal stereotype of horrors, this therefore makes the horror different and unfamiliar. On the other hand sometimes the first victim will be a young child which creates shock for the viewers and therefore want the hero to prevail. In many horrors the police will be ineffective, thus allowing the horror to continue. Often they will not believe the stories they are being told by people who have witnessed these terrifying things.

Themes
There are many themes which a horror could contain. Some are binary oppositions such as natural vs unnatural or good vs evil or known vs unknown. An example of this could be war of the worlds where the humans fight against the unknown aliens who try to take over the world. Automatically we will take the side of the humans as they are our kind. It is often easy to tell what side of the conflict we as viewers want to prevail. Furthermore, the return of the repressed or Freudian theory represents that horror is close to sex in some way.
Occasionally, the evil will have hidden evil inside them which is motivating them to do these evil things or go against society. Also, a horror may be themed by the ideology of what lies on the other side of death. This may contain a dead villain torturing people who are alive in a quest to come back to life. The theme of a horror could also be revenge where the villain wants to kill people because of what they have personally suffered.