Shelley's Frankenstein begins with a letter that Robert Walton (the explorer who is the main narrator of the story) is sending to his sister. He begins by describing the exploratory journey that he plans to take into the arctic north. He also tells his sister that he is longing for a friend -- someone that is a kindred spirit to him.
We soon learn that he has met a man named Victor Frankenstein -- a man with an extraordinary story. Walton feels a kinship with Frankenstein, and as Frankenstein tells Walton his story, Walton writes the story down.
So, the novel is "framed" through Walton's point of view, as we have Frankenstein telling the story to Walton, and Walton relaying the story to his sister. (Later, we get the Creature's story as well...).
The novel begins with a series of letters from Robert Walton to his sister, Margaret Saville. When a story is told through letters, this is called an epistolary narrative/novel. (Below is a link from wikipedia that explains this further). The one thing about this style is the way it impacts the personal quality of the narrative. You are viewing a private conversation between a brother and a sister. Because of this, there is a sense of voyerism (observing something uninvited). There are things revealed therein that would not be for public eyes. Think of it kind of as if the author is allowing you to peak inside the closet to view the family skeletons. It also gives the sense of learning of events from a primary source (a witness) rather than second hand. This is kind of an illusion where Frankenstein is concerned because we are told, but seem to forget quite quickly, that Robert is retelling a story told to him. It seems in fact that the narrator becomes Victor Frankenstein, but in reality this creates a paradox because what seems to be an eye witness account is, in reality, a retelling of events. He told me/I am telling you.