First of all you should have asked me this question in the categories Greek or Literature, as it has little to do with history.
Anyway, although it is a very broad question, I’ll try to make a brief summary about Aristotle's views on tragedy.
In his 'Poetics' (written in the late 4th Century BC) Aristotle, who was the first to write a book on how to analyse plays, uses Sophocles’Oedipus the King as a paradigm of a perfect tragedy and says that ”tragedy is an imitation [Greek, ‘mimesis’] of an action that is serious, complete and of a certain magnitude ….through pity and fear effecting the proper purgation [Greek ‘catharsis’] of these emotions".
In fact he thinks that this purification brings about spiritual renewal or release from tension.
Moreover he lists 6 principal elements in the structure of tragedy, i.e. 1) plot (Greek ‘mythos’2) character, 3)theme or thought, 4) diction, 5) music, and 6) spectacle, and says that tragedy is an imitation not of men, but of an action and of life, and life consists in action.
In short Aristotle considers the plot to be the soul of a tragedy, with character in second place.
The goal of tragedy is thus not suffering, but the knowledge that issues from it, as the denouement issues from a plot.
Also, Aristotle thinks that the tragic hero must be neither a villain nor a virtuous man, but a "character between these two extremes, a man who is not eminently good and just, yet whose misfortune is brought about not by vice or depravity, but by some error or frailty [Greek ‘hamartia’]".
This ‘hamartia’ is produced by the 'hybris' (literally, arrogance, overweening behaviour) which is nothing but 'excessive pride and presumption of the protagonist suggesting impious disregard of the limits governing human action in an orderly universe'.
Finally, the most powerful elements of emotional interest in tragedy, according to Aristotle, are reversal of intention or situation (Greek ‘peripéteia’, i.e. a turning point, a change from good fortune to bad fortune) and recognition scenes, so that the hero saw his destiny suddenly reversed from happiness to misery.
Hope this outline can give you a first insight on this interesting matter which however is quite difficult.
