There are three different narrators in "Frankenstein".  The effect of this method of storytelling is that the reader is provided with an understanding of events from multiple points of view.

The first narrator introduced is Robert Walton, who, as a neutral party is able to give the reader a sense of objectivity and reliability.  The author, Mary Shelley, uses a device called "epistolary form", in which Walton relates what has happened through a series of letters written to his sister.  Walton's letters appear at the beginning and the end of the narrative, framing the main body of the story which is told by Victor Frankenstein and the monster.

Victor Frankenstein is the second narrator presented in the book.  He gives us background on his own childhood and upbringing, and the events which led to his fateful creation of the monster.  The monster himself then interrupts Victor's narrative to tell his own story, after which Victor once again resumes the tale, describing what transpires until the very end, when the narrative is returned to Walton, who provides the conclusion.