A ‘voice over’ is simply a recorded, spoken message. Voice overs are used to communicate messages to listeners and are used effectively in a wide range of contexts. When you are at the cinemas and hear a booming voice advertising the latest releases, you are hearing a voice over. When you are put on hold and are listening to a voice telling you about the latest offers, you are listening to a voice over.
In the world of jingles, voice overs are used to deliver messages that are relevant to the time in which they are being heard. A jingle has longevity. Most businesses will not change their name once it has been established. When they get a jingle made, they are likely to stick with it for years (perhaps forever). This is all because of familiarity. They want people to remember their name, their jingle and the core message of who they are. What does change however, is the current message that they wish to convey. For example, a business may have a sale on for the next week only, or a new product may be released to coincide with a current event (a new movie for example). This is where voice overs are effective. A voice over can easily be recorded and used with a jingle (usually a top and tail) so that the message delivered is current and relevant, but the aural branding of the jingle is still present and effective.
For more definitions, read the post jingle lingo, what does it all mean.