The Narrator was Earth's guide through The Twilight Zone.
The Narrator appeared to possess near omniscience, most notably in the awareness of the convergence of dimensions and the resulting effects. His—there have been no female narrators as yet—appearance changed, along with his personality, from time to time. It was never determined if these were merely guises or if they were in fact separate entities fulfilling the same role. Assuming the latter, there have been at least seven distinct individuals assuming the identity of The Narrator, though only two have been visibly observed.
The Narrator appeared to be documenting the events he witnessed on Earth as it encountered the Twilight Zone. It remained unknown exactly why or for whom he had chosen to record these incidents, but it appeared from the nature of his narrations that he had a message of morality to deliver.
This was the first incarnation of The Narrator known to exist. First only materializing as a voice, he was later revealed to be a suave, amicable man whose astute observations gave special insight into the people and events that he introduced. (TZ1)
Dressed most often in a mod jacket and tie, he often appeared at the location of the event that he was observing and even once clearly interacted with the person involved. In this instance, one of the individuals in the background, Gregory West, referred to The Narrator as "Rod" and seemed to possess not only the ability to observe The Narrator himself, but to possess some control over the visitor's manifestation in "his" realm. It was unclear to the extent to which this was true and it was possible The Narrator chose to leave voluntarily, as The Narrator continued to deliver his monologues on Earth for quite some time afterward. (TZ1: "A World of His Own")
This incarnation possessed a great sense of humor as well as a great sense of morality, though not above sternly judging the persons in the tale for their perceived follies, flaws, and immorality. He had a taste for cigarettes and could occasionally be seen smoking one.
Nothing has been disclosed about the possible existence of any other narrators before this incarnation.
This narrator never appeared in person, but relayed his tales orally. Of all of the narrators, he appeared to have had the briefest contact with humans. (Twilight Zone: The Movie)
Like the prior incarnation, he never made himself visible to the people of Earth. His voice was that of a comforting, warm man, differing from the rugged tone of the previous Narrator and sounding slightly older than the first. He also didn't seem as willing to place judgments on the individuals he observed, although he, too, pointed out the moral implications of each account. (TZ2)
As with the two previous narrators, this incarnation remained invisible. (TZ2)
This Narrator was only the second to allow his presence to be visible by Earth people and the first verified guide through the Twilight Zone to not possess the appearance of a Caucasian. He dressed fashionably, but in a much more casual manner than the First Narrator. He also didn't seem to be as jovial, yet was not an unkind or unlikable individual. Perhaps this was because the stories and individuals he introduced tended to be more serious, suspenseful, and dark in nature. It is unknown if this was his choice or merely a sad reflection on the progression of modern society. (TZ3)
On the production side of things, Rod Serling was not the first actor to fill the role of "The Narrator." The job had originally been secured by Westbrook Van Hoorhis, but a decision was made before production started on "Where is Everybody?" to replace him with a reluctant and inexperienced Mr. Serling. His demeanor and appearance in the opening sequences of the series would become a well-referenced hallmark of The Twilight Zone.