According to Aristotle, both tragedy and epic are forms of imitation. Tragedy imitates directly by means of mimesis and epic both directly and indirectly by means of mimesis and diegesis. While epic imitates solely by means of words, tragedy also imitates by means of spectacle. Both epic and tragedy portray people better or greater than the average spectator, unlike comedy which portrays people as worse than they actually are and uses humour to dissuade us from acting badly (Aristotle's lost treatise on comedy may be summarized in the Tractatus Coislinianus; see also Eco, The Name of the Rose for interesting view of Aristotle on comedy)
Due to practical constraints of production, tragedy ideally has a single unified action which is often restricted to a specific time and place. and a small n umber of characters, while epic has a broader scope of coverage.