How we change what others think, feel, believe and do
The narrator tells the story or otherwise explains things that are not particularly evident from the action within the story.
In aural and written storytelling, the storyteller or author is usually the narrator, although the device of a character speaking to the reader is sometimes used.
Narration becomes more difficult in visual storytelling on the stage or screen. In this case a disembodied voice may be used, perhaps a person from the story or a 'full-time' narrator. On the stage the narrator may stand at the edge of the stage. When actors who are present narrate, it is often in the form of an aside.
The TV series 'Desperate Housewives' uses a woman who died but knew most of the participants to narrate the action.
'Reader, I married him' - from Jane Eyre.
In visual action what people are thinking may be significant. Other detail may also not be easily shown but quite easily explained in a few words. In these cases a narrator is a useful device.
Narration on stage is rare -- in visual presentations it comes into its own, however, on TV and in movies, where a voiceover is often used to explain detail.
Narration is common in non-present devices such as the flashback, where the person may be narrating in the present about an event in the past that is being shown.
An omniscient narrator knows everything, god-like, even the private thoughts that people are thinking.
An unreliable narrator makes mistakes or adds personal bias into the storytelling. They usually tell the story from the first person perspective, which portrays them as normal and thus legitimizes to some extent their very human lack of reliability.
Telling a story as opposed to showing or acting it is called Diegesis. Showing the story is called Mimesis.