Chorus, in drama and music, those who perform vocally in a group as opposed to those who perform singly. The chorus in Classical Greek drama was a group of actors who described and commented upon the main action of a play with song, dance, and recitation. Greek tragedy had its beginnings in choral performances, in which a group of 50 men danced and sang dithyrambs—lyric hymns in praise of the god Dionysus. In the middle of the 6th century BC, the poet Thespis reputedly became the first true actor when he engaged in dialogue with the chorus leader. Choral performances continued to dominate the early plays until the time of Aeschylus (5th century BC), who added a second actor and reduced the chorus from 50 to 12 performers. Sophocles, who added a third actor, increased the chorus to 15 but reduced it to a mainly commentarial role in most of his plays (for an example of this role as shown in the play Oedipus the King, see video) . The chorus in Greek comedy numbered 24, and its function was displaced eventually by interspersed songs. The distinction between the passivity of the chorus and the activity of the actors is central to the artistry of the Greek tragedies. While the tragic protagonists act out their defiance of the limits subscribed by the gods for man, the chorus expresses the fears, hopes, and judgment of the polity, the average citizens. Their judgment is the verdict of history. (Source: Britannica)

Read more about the function of the chorus in Greek drama.

People in ancient Greece often thought of the world as being a fight, or an agon, between the two forces of rationalism and chaos, or between law and nature. The Greeks called these two forces “nomos”, meaning law and order and rationalism, and “physis” (FU-sis), meaning nature (our word physics comes from this: physics is the study of nature). Chaos is what there was before there were gods; the gods came to bring order to the world, and the gods like order and hate chaos. (Compare this to the Hindu idea of dharma, equally descended from earlier Indo-European ideas, and then compare both of these to the Chinese idea of the Tao.) Pretty much everything was either on the side of nomos (order) or on the side of physis (nature).

The god Apollo was the representative of nomos even among the gods; he was always fighting snakes and dragons and sea monsters, who represent physis for the Greeks. The snake-woman Medusa, who cannot control her own behavior even inAthena‘s temple, is an example of physis.

Women in general tend to be (in the view of Greek men) on the side of physis, while men are generally on the side of nomos: men can control themselves while women become hysterical (the word is Greek and it means to be taken over by your uterus). Men make and obey laws, but women do what comes naturally (like Phaedra or Medea, or the maenads in the Bacchae). How does this compare to the Chinese idea ofyin and yang? Mathematics, music, and architecture, which all follow strict sets of rules and attempt to order nature, are all good examples of nomos. Architecture brings space under control, music brings noise under control, and mathematics brings the infinite under control. For this reason, Greek men liked all three of these arts very much. It’s possible that women express nomos by spinning and weaving, which also create order out of chaos. They take a mess of fleece, and transform it into useful thread and blankets and clothing.  (Source)

The basic structure of a Greek tragedy is fairly simple. After a prologue spoken by one or more characters, the chorus enters, singing and dancing. Scenes then alternate between spoken sections (dialogue between characters, and between characters and chorus) and sung sections (during which the chorus danced). Here are the basic parts of a Greek Tragedy:

a. Prologue: Spoken by one or two characters before the chorus appears. The prologue usually gives the mythological background necessary for understanding the events of the play.

b. Parodos: This is the song sung by the chorus as it first enters the orchestra and dances.

c. First Episode: This is the first of many "episodes", when the characters and chorus talk.

d. First Stasimon: At the end of each episode, the other characters usually leave the stage and the chorus dances and sings a stasimon, or choral ode. The ode usually reflects on the things said and done in the episodes, and puts it into some kind of larger mythological framework.

For the rest of the play, there is alternation between episodes and stasima, until the final scene, called the...

e. Exodos: At the end of play, the chorus exits singing a processional song which usually offers words of wisdom related to the actions and outcome of the play. (Source)