Use of Narration

Narration in films is a voice-over which briefly tells the story of what shall happen in the movie. Also, it can be used in a variety of different film genres, here are a few examples:

1) Narration can be seen in the opening of "Kick-Ass"-

Genre: Action

Narrative here is used to introduce what the film is going to be about. 
"At some point in our lives we all wanted to be a superhero" makes us aware that during this film somebody will become a superhero and will try to save other people.

2) Narration can also be heard in the Disney film "Dinosaur" -

(skip to 3:01 for narrative)

Genre: Adventure

Narration in "Dinosaur" has been used differently than in "Kick-Ass". Rather than having a narrative at the very beginning of the movie this Disney film chose to use it at the end. This has been done to create a sense of closure for the viewers rather than to introduce the story itself.