Digetic & Non-Digetic Sound

Diegetic Sound
This is sound that is visible on the screen of a film or sound made by something that is implied present, for example:
- People Talking
- Sounds made my objects
- Music created by instruments in the story

Here is an example including diegetic sound from one of my favourite films: '17 Again' 

Non-Diegetic Sound
This is sound which is not visible screen or implied to be present in the film, for example:
- Commentary (by a narrator)
- Sound effects added for dramatic effect
- Mood Music

Here is an example including Non-Digetic sound in another of my favourite films: 'The Lucky One'. The majority is mood music.