Different types of sound in the world of film

In the world of film, there are many different types of sound, but the main two distinctive types of sound in this industry are diegetic and non-diegetic sound.

Diegetic sound is a term used to describe all types of sound that is visible to the reader, ie the audience will know where the source of the sound is coming from, eg in one of the episodes of EastEnders,  it is visible that a kettle in a kitchen is turned on and is boiling water, the sound at which the kettle makes is diegetic due to the audience knowing where the source of the sound is coming from (the kettle).

Now the opposite of diegetic sound (sound at which the audience knows where the source of the sound is coming from), is non-diegetic sound-this is a term used to describe  any type of sound that is not visible to the audience, ie the audience does not know where the source of the sound is coming from.
Common examples of non diegetic sound are voiceovers- we do not know where the source of the sound is coming from, also the music which starts in the opening credits, again we are unaware where the source of the sound is coming from.