Sound

In its simplest form sound can be broken down into two categories

Diegetic - sound which occurs naturally within a scene (e.g. dialogue,sound effects)
Non - Diegetic - sound which has no obvious source and is added on (e.g. Jaws theme during and attack)

Within these categories there is however important distinctions to be drawn.

Diegetic -Dialogue. An actors voice can add important information about a character just by varying pitch and tone.
Synchronous sounds are those sounds which are synchronized or matched with what is viewed. For example: If the film portrays a character playing the piano, the sounds of the piano are projected.

Synchronous sounds contribute to the realism of film and also help to create a particular atmosphere. For example: The “click” of a door being opened may simply serve to convince the audience that the image portrayed is real, and the audience-may only subconsciously note the expected sound. However, if the “click” of an opening door is part of an ominous action such as a burglary, the sound mixer may call attention to the “click” with an increase in volume; this helps to engage the audience in a moment of suspense.
Asynchronous sound effects are not matched with a visible source of the sound on screen. Such sounds are included so as to provide an appropriate emotional nuance, and they may also add to the realism of the film. For example: A film maker might opt to include the background sound of an ambulance's siren while the foreground sound and image portrays an arguing couple. The asynchronous ambulance siren underscores the psychic injury incurred in the argument; at the same time the noise of the siren adds to the realism of the film by acknowledging the film's (avowed) city setting.
Non-diegetic sound
Background music is used to add emotion and rhythm to a film. Usually not meant to be noticeable, it often provides a tone or an emotional attitude toward the story and/or the characters depicted. In addition, background music often foreshadows a change in mood. For example, dissonant music may be used in film to indicate an approaching (but not yet visible) menace or disaster.
Background music may aid viewer understanding by linking scenes. For example, a particular musical theme associated with an individual character or situation may be repeated at various points in a film in order to remind the audience of salient motifs or ideas.
General terms
Sound bridge. Adding to continuity through sound, by running sound (narration, dialogue or music) from one shot across a cut to another shot to make the action seem uninterrupted.