The Poetics (Greek: Περὶ ποιητικῆς, Latin: De Poetica) is a treatise by Aristotle on literary theory. Like many of his other works, it exercised a formative influence on the discipline it examined up to the renaissance.
Aristotle divided poetry into tragedy and comedy and into narrative and dramatic forms. Like Plato, Aristotle saw the essence of art in representation or mimesis. In contrast to the critical view of mimesis in Plato's dialogues, Aristotle's theory of catharsis suggested that tragedy could have a positive effect through purging negative emotions.
Free online texts
Gutenberg: The Poetics, translated by S.H. Butcher. Multiple formats.
University of Adelaide (Internet Archive): Poetics, translated by S.H. Butcher. EPUB, HTML and MOBI formats.
BBC In Our Time: Aristotle's Poetics, radio broadcast presented by Melvyn Bragg.
History of Philosophy without any gaps: Stage Directions: Aristotle's Rhetoric and Poetics - podcast by philosopher Peter Adamson.
Internet Encyclopedia of Philosophy: Aristotle - Poetics, by Joe Sachs.
Librivox: The Poetics - public domain audiobook.
Wikipedia: The Poetics (Aristotle).
The Great Conversation: further reading at Tom's Learning Notes.
Ancient Greek resources: Learn to read Greek classics in the original.
Bloom's Western Canon: The Poetics is listed.