Sound Within Film

There are two main types of sound within film, diegetic and non-diegetic.
Diegetic sound is sound from within the world of the text. For example, the dialogue between two characters would be classed as diegetic sound.
Diegetic sound can then be broken down into on-screen and off-screen sound; these are rather self-explanatory. On-screen sound is sound that we can see being made, such as the protagonist of a film walking; we can hear the footsteps, and also see the character making them. Off-screen sound is when the sound is being made from somewhere other than the place we can see. The sound, however, is still being made from the world of the text. An example of this is the sound of wildlife.
Non-diegetic sound is the sound that is not from the world of the text, for example narration or background music.
The terms diegetic and non-digetic are not limited to sound; they can be applied to many elements of filmography.