We all are familiar with voice-over as every one of us has heard the commentary of a sports match on a radio and broadcasting a non-diagetic voice is called voice-over.
A sound is called diegetic when it’s a part of the current scene for example, if a sound is played like a guitar is being played and the music from the guitar is heard by the people in the scene then the voice or sound is called diagetic. However, when the sound is not heard by the current people or characters in the scene, the voice is called non-diagetc. Voice-over can be used in a live show by some one on a radio but voice-over does not mean that the broadcast has to be live, voice-over just refers to broadcasting the non-diagetic voice irrespective of whether the voice is recorded or live.

Voice-over finds its applicability in television news where different scenes of different situations are shown together and there is reporters describing everything by voice-over facility. Voice-over is also used in commercial advertisements and also in reality shows where the winner might be awarded a voice-over.

Voice-over can be used as a creative device to give voice and personality to animated characters. Voice-over is also used in movies to show ironic counterpoint and also it can be used to show flashback of a person in a movie. It’s also used to give effects like voices which are not directly connected to the current scene.