The "Poetics" contains Aristotle's observations on what elements and characteristics comprised the best tragedies based on the ones he'd presumably seen or read. He divides "poetry," which could be defined as imitations of human experience, into tragedy, comedy, and epic, and explains the differences between these forms, although comedy is not covered in detail and tragedy gets the most treatment. For one thing, tragedy, he states, seeks to imitate the matters of superior people, while comedy seeks to imitate the matters of inferior people.
To Aristotle, the most important constituent of tragedy is plot, and successful plots require that the sequence of events be necessary (required to happen to advance the story logically and rationally) and probable (likely to happen given the circumstances). Any plot that does not feature such a necessary and probable sequence of events is deemed faulty. Reversals and recognitions are plot devices by which tragedy sways emotions, particularly those that induce "pity and fear," as is astonishment, which is the effect produced when the unexpected happens. He discusses the best kinds of tragic plots, the kinds of characters that are required, and how their fortunes should change over the course of the plot for optimum tragic effect.
With regard to poetic language or "diction," he emphasizes the importance of figurative language (metaphor, analogy) in poetry and the importance of balancing figurative with literal language. It is his opinion that metaphoric invention is a natural ability and not something that can be taught. Of all the poets Aristotle mentions who exemplify the ideals proposed in the "Poetics," Homer draws the most praise.
Malcolm Heath's introduction in the Penguin Classics edition offers some helpful and amusing clarification and commentary on the "Poetics," including a demonstration of the Aristotelian method of constructing a tragedy using the story of Oedipus as an example. A work that is scant in volume but rich in ideas, the "Poetics" demands to be read by all those interested in ancient thought on literature.
I feel that this is a horrible translation of an otherwise great work of literature. This translator felt the need to re-arrange pieces of Aristotle's work, and completely relocate some to an "Appendix." If you find this appalling, then you need to find another translation. However, if you are fine with the butchering of another person's work, by all means, order this book.