Aristotle is not only one of the first theatre theorists but he is also one of the most influential, although his ideas have not always helped dramatists create effective theatre especially as they were widely misinterpreted during the Renaissance.. In The Poetics Aristotle sets out his theories about Tragedy and invents some terms which have become widely accepted. He draws distinctions between types of narrative, particularly between Epic and Tragic.
This is his definition of tragedy:
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; through pity and fear effecting the proper purgation of these emotions.
Because of his importance as a philosopher there is a huge amount of information about Aristotle available. A useful starting point is the University of Hawaii whose site gives a detailed account of each section of The Poetics as well as explaining the meaning of the terms which Aristotle invented.