- Romeo and Juliet begins with a Chorus who frames the story using a introductory sonnet as a preview of the coming action.
- The actor is a male, since all actors were male in Shakespeare's day.
- The actor is alone on stage. The Greeks used many members in their Choruses, but Shakespeare limits it to only one so as to focus on the plot and tragic heroes.
- The Chorus reveals the main themes of the play: love, death, hate.
- He even says how long the play will be: "two hours traffic of our stage." In this way, he is like a preview that you might see in the theaters.
- He speaks beautiful rhymed poetry, elevated language, to highlight the elevated themes of the play.
- He speaks in a sonnet, which is usually divided into two parts (octet: first 8 lines and a sestet: last 6 lines). This problem-solution format mirrors the tragic structure of the play.
The "chorus" in any major dramatic production largely serves the same functions:
- narrator: gives necessary background at the beginning of scenes and sums up or reviews them at the end
- MC: in Shakespeare's time there were no special lighting effects or microphones to get the audience settled - the chorus took care of this
- music: not actual music - but just like in Sophocles' plays, the chorus acts as a "musical" (and often comical) interlude. In R&J the chorus' prologue is written as a sonnet - which provides a musical rhythm and rhyme
- audience appeal: the chorus often mirrors the sentiments of the actual intended audience at the time the play was written. It was a way for playwrights to say "I'm on your side," and broaden their popularity with 'commoners' - it was also a way to express the often humorous cultural view of the events presented. It gives US a better perspective of the social setting of the times.
The link below is a previous discussion thread you might also find helpful.
I assume that you do not mean that you need a physical description, because there is nothing to tell us anything about how many people are in the chorus, whether they are men or women, or anything else like that.
I would assume that the chorus is made up of people from Verona since they know more than us about the setting. Their purpose is to let us know more or less what is going to happen in the play. Because we know the "star-crossed" lovers' story will end badly, all of their actions seem more poignant and sad. That's because we know that all their efforts are doomed.