Diagetic

Diagetic sound is sound that is coming from the scene, the source doesn't have to be visible from the camera if the audience know that the source is in the scene. The sound coming from the actors and objects on in the scene are diagetic. An example of diagetic sound is music coming from a radio that was turned on by the character. In the film Top Gun, Tom Cruise reacts to the music from within the world of that story (the jukebox).
Non-diagetic sound is the opposite to diagetic sound. Non-diagetic sound is sound that isn't from within the scene. Sound that has no source from within the scene is non-diagetic. An example of non-diagetic sound is the characters thoughts playing without the character talking on the screen. In the film Batman Begins two character are talking and music can be heard but the characters cant hear it because its not in the world of the story.
Mood and meaning is how the audience should be feeling during the scene. Music can effect the mood and meaning for example, if the film is an action film it will have fast paced music, it its a comedy it will have upbeat music. In the film The Bourne Identity during a short fight scene the music jumps in and is fast paced like the action.
Juxtaposition is sound and video that contrast with each other. This can be used for a comic effect. An example is during a murder, happy music is playing. This is mixing something bad with something happy making it funny. At the beginning of the film Funny Games metal music is playing while a family is driving in the car, the image and the sound are opposites.