The Narrator, voiced by Ron Howard, narrates the events of Arrested Development from a third-person perspective. Ron Howard is not directly credited for the role, but he does voice the narrator in all 76 episodes.
The narrator could be considered a "Detached Observer" type of narrator, as he is no character directly involved in the story and never speaks of himself. However, he occasionally does interject his own opinions on the bizarre acts of the Bluth family via his intonation and interjections and could be considered a "Commentator" type of narrator.
Although initially just guiding the audience through the fast-paced storytelling of Arrested Development, the narrator eventually gains a personality and interjects his own commentary. At times he insults members of the Bluth family such as G.O.B., as seen in "Motherboy XXX" and "Family Ties" and Buster in "The Immaculate Election". The narrator also takes pride in his narration, going so far as to insult the Scandalmakers narrator in "Spring Breakout".
Connection to the Bluths
It is never clearly stated how the narrator is related to the Bluth family. It is intentionally left ambiguous whether the Ron Howard character that appears in "Development Arrested" and Season Four is supposed to be the narrator of the series or if it just a coincidence. On a couple of brief occasions, it is suggested that Ron Howard is the narrator, as seen in "Fakin' It" when a hand with an "RH" ring turns the page of a magazine and in "Exit Strategy" when George Michael sends Ron Howard an invitation to Maeby's 16 birthday party and the following line of dialog occurs:
Maeby: So, did you send [an invitation] to everyone in my address book?
George Michael: Yeah. I’m really sorry they didn’t come. I don’t know what happened.
Maeby: I do. They found out I was 16.
Narrator: And a lot of us just didn’t want to drive down to Orange County.From "Exit Strategy"
Season Four does not offer a straight answer to the question, as the Narrator on one hand constantly refers to the on-screen Ron Howard in third person, but on the other hand always tries to paint Ron Howard, and his daughter Rebel Alley, in as positive a light as possible.
Ron Howard interview
James Poniewozik: Without spoiling anything you don’t want to spoil, can you talk a little bit about your acting role in the new season?
Ron Howard: I’m not a central figure. But I am a plot point. And I particularly enjoy that the narrator is oblivious to who is Ron Howard. He couldn’t care less who this Ron Howard person is. Just another specimen to be examined. 
Jessie Bowers: [to George Michael] Daddy lost his shot at happy, and it's all your fault, Opie.
Narrator: Jessie had gone too far, and she had best watch her mouth.From "Public Relations"
Michael: Okay, that would be disgusting if you’d actually slept with her, but I don’t think you did.
G.O.B.: I did. And it was disgusting.
Narrator: They didn’t, but it would have been.From "Motherboy XXX"
Scandalmakers Narrator: When allegations surfaced that he’d illegally built homes in Iraq, George Bluth Sr., assisted by his secretary, Kitty, escaped the country...
Narrator: Due to poor acting, the burden of the story was placed on the narrator.
Scandalmakers Narrator: ...only to be found hiding under one of the homes that he himself built.
Narrator: He was actually found in a hole near the house, but this inattention to detail was typical of the laziness the show’s narrator was known for.
Tobias: [As George] I cannot survive under the house. Perhaps an attic shall I seek.
Narrator: Real shoddy narrating. Just pure crap.From "Spring Breakout"
G.O.B.: Worth every penny. Even you have to admit that that was the lamest thing ever put on tape.
Narrator: Not so. Buster had once videotaped himself recreating moments from his then favorite film, Chicago.From "The Immaculate Election"
Narrator: Now that's a clear cut situation with the promise of comedy. Tell your friends!From "S.O.B.s"
Michael: And you did things with [Nellie]?
Narrator: He didn’t.
G.O.B.: I did.
Narrator: Well, if you consider crying like a girl doing something.From "Family Ties"