A
voice-over is a dialogue track which is delivered by a voice-over actor in conjunction with existing
video or
audio. Voice-overs are non-
diegetic, a technical term meaning that they are not part of or a result of the 'action', they exist behind
the fourth wall. The most common use for a voice-over is to commentate on
video footage, usually for a
commercial, as
narration for a film, or for a television
documentary. Performances for cartoon films and dubbing of foreign-language actors are not usually considered voice-over work, although there is no technical and very little artistic difference.
Voice-over actors are typically known as 'voices'. Famous voice-over actors include Orson Welles, Don LaFontaine (the 'movie trailer voiceover man') and Leonard Nimoy, all of whom are or were possessed of deep, grand-sounding voices.
In the UK, a great deal of RADA-trained Shakespearean actors spend much of their time recording voiceovers for commercials, as there are few opportunities for them to perform otherwise - and the money is good, too. In the UK, the most popular celebrity voiceover people (in terms of the amount of work they are given for television programmes and adverts) are or have been Ian Holm, John Hurt, Stephen Fry and the late Bob Peck. Bob Peck was the most common voice of 'Horizon', the documentary series, for example.
If you want to be a voice-over artist yourself, you will need to develop the ability to deliver a script resonantly, convincingly, repeatedly, and accurately, even if the script is poorly-written. That's all there is to it, essentially.