In his Poetics, Aristotle says that tragedy is the greatest of literary art forms because its agents and devices are better than us (the reader/audience). Whereas comedy is more realistic, even below us in its aims, tragedy leads man to question the moral choices and responsibilities of being a human and, therefore, enters the religious and ethical realms.
His definition of tragedy is as follows:
Tragedy, then, is an imitation of an action that is serious, complete, and of a certain magnitude; in language embellished with each kind of artistic ornament, the several kinds being found in separate parts of the play; in the form of action, not of narrative; with incidents arousing pity and fear, wherewith to accomplish its katharsis of such emotions. . . . Every Tragedy, therefore, must have six parts, which parts determine its quality—namely, Plot, Characters, Diction, Thought, Spectacle, Melody.
So, the embellished language (the poetry) must be elevated, higher in purpose and more beautiful than everyday speech. Since early Greek tragedy was religious in nature, its language was a form of praise. It is much like a combination of a poetry, sermon, and hymn from a church service.
The chorus is the ideal reader/audience in Greek tragedy. Through their melody (embellished language, poetry, choral odes), the chorus not only informs the audience how to act or respond to the action of the play, but they give unity to the plot. Since they are always on stage, and since the audience is always watching, they are an intermediary between the actors and audience.
In the end, the embellished language by the actors and chorus must move the reader/audience toward a katharsis, a purgation of pity and fear. The audience must pity the tragic heroes and fear that the tragic heroes' fate may be their own.