A complete translation of the Poetics can be found here.
Aristotle's Poetics, written around 350 BCE, consists of 26 surviving chapters focusing mainly on epics and tragedies (his work on comedy has sadly been lost). Aristotle distinguishes between rhetoric and poetry by claiming that "Socratic dialogues" (dialectic or rhetorical discourse) cannot rightly be compared with poetry. The only real connection is that they both use language. He goes on to say, rather beautifully, that it's "not the [use of language] that makes the poet, but the verse that entitles them all to the name."
Aristotle defined tragic and epic poems by the same general categories: simple, complex (or ironic), ethical, and passionate (or "pathetic"). He also details the six essential elements of tragedy: plot, character, thought, diction, spectacle, and song, with plot being the most important element -- the "soul" of the tragic poem. Next, he outlined the four essential qualities the protagonist of any good poem will have. According to Aristotle, a true hero will have a strong moral sense and purpose; a hero will be valiant and brave; a hero will be somewhat realistic in his or her abilities; and a hero will be consistent in his or her decisions and actions.
Applications of Theory
This page is going to briefly outline the Poetics. The book is divided into several "books" that will each be briefly summarized here.