Sound

Diegetic sound - The source of the sound is in the film world e.g. a character drops a glass. This creates the effect of a 3D world.
Non-diegetic sound - The source of sound is not in the film world e.g. music for effect. This can manipulate an audiences response emotionally and show something that is not visible. It can also create an atmosphere.
Sound bridge - When a sound moves from one shot to another. This can help with flow and cohesion.
Parallel sound (synchronous) - Goes together with the scene e.g. happy music with a happy scene. This can add to the atmosphere of the scene and create the right mood.
Contrapuntal sound (asynchronous) - The music doesnt go together with the scene e.g sad music with a happy scene. This either makes a specific point or can be used for comedic effect. this can be seen in Trainspotting when Carmen by Bizet is being played at the beginning as it does not fit in with the scene but just adds effect to it.
sound-motif - A specific piece of music repeated in films that everyone knows e.g. Jaws, James Bond, Indiana Jones.
Synaesthesia - This is what you see matches the shaoe if the action.

The reasons for using sound can be to manipulate audiences responses to certain things, for visceral pleasure (a physical response) or show things that cant be shown on screen in a kids film and leave the audience to come up with their own interpretation of the sound.