From a literal standpoint, a voice over artist is a talent who is reading, or working from memorized material, otherwise known as a script.
In other words: you hear the person's voice, but you do not see them. If you ask the average person what comes to mind when they hear the words ‘voice over,’ more often than not the answer is radio and TV commercials.
Commercials are indeed one component of the voice over field. Any time you hear a voice over artist in an advertisement without seeing the person attached to that voice, you are listening to a commercial voice over.
What most people fail to realise is that there is a broad range of commercial opportunities, many of which do not demand the typical used car salesman type delivery. For example, voice overs for socially relevant material, public service organisations, iphone and ipad apps, e-books, medical narrations, are all examples of commercial voice over work that voice over artists perform.
Similarly, children’s products, upcoming community events, and material related to health care all fall under commercial work recorded by professional voice over artists.