Who narrates the prologue, and what is its purpose?
I have always read it as Chaucer, the traveler (he was well-traveled and educated). In addition to the other excellent answers you have received, the prologue serves to introduce the theme of hypocrisy. Chaucer goes to great pains throughout the tales and prologues of the tales to show the dishonesty of clergy. By taking what should be a soul-cleansing and cathartic journey to the resting place of St. Thomas a Becket and making it into a contest of story telling (some of them quite dirty...check out the Miller's tale), to which ALL the pilgrims agree, the spirituality of the pilgrimmage is diminished. This sets the tone for the rest of the poem.
The narrator of the Prologue might be Chaucer, but this is not to confuse him with Chaucer the author. He explains, in fact, that he is only acting as the faithful reporter of what others have said, without adding or omitting anything; he must not then be blamed for what he reports. "My wit is short, ye may well understand," the narrator says. This persona, who almost becomes a character in himself, often professes a naivety that we often find in his ironic descriptions of the pilgrims. Often that narrative voice presents information uncritically but ironically in that the author behind the narrative voice does the criticizing.
In addition, the prologue establishes the contest with the stories that are told, and it gives background on each of the characters. The background becomes important when reading each of the individual tales. The narrator tries to establish himself as someone who is impartially relating events and the stories, but because it is Chaucer, a keen reader realizes he is responsible for the bawdy content although he tries to beg off responsibility.
The narrator is Chaucer, the author of the poem, although he is not actually named.
The prologue sets the stage for the rest of the poem, providing structure and context. In the prologue, twenty-nine travelers meet at the Tabard Inn in London, in preparation for a journey to the Shrine of St. Thomas a Becket in Canterbury. The poem is made up of the stories they tell along the way.