Poetics is Aristotle's classification of poetry. He classifies the different types of poetry and goes over various conventions of poetry and how it can be done well and how it can be done poorly.
"Poetry", as Aristotle defined it, referred both to what we would think of as poetry (i.e. written verse) and also plays, mainly tragedy and comedy. Poetics deals mainly with tragedy and epic poetry. Aristotle wrote a second book about the remaining forms of poetry (e.g. comedy), but it has been lost. Aristotle wrote that poetry is primarily a mode of imitation, which is different, I think, from modern views of poetry. We would probably say poetry, or art in general, is primarily self expression. We tend to shy away from calling art "bad" by saying things like "beauty is in the eye of the beholder" and "to each his own", but in ancient Greece, art either hit the mark (was a good imitation) or it didn't (was a bad imitation). Like a lot of Greek thought, I find these ideas refreshing and I would be inclined to agree with them. I would love to see an ancient Greek philosopher like Plato or Aristotle walk into a modern art gallery and debate with modern artists and critics. It would make my day.
Even though this book was aimed at tragedies and epic poetry, much of the guidelines Aristotle sets out could be readjusted to fit modern movies or novels. For example Aristotle's listing of the six components of a tragedy (Spectacle, Character, Plot, Diction, Melody, and Thought) seem like they could be applied, with some adjustment, into most forms of story-telling.
I prefer epic poetry to tragedies, but I have to admit, Aristotle gave really good reasons for the superiority of tragedies. He said that tragedies can be enjoyed being watched or being read, while epics can only be read and tragedies relative short length make them pithy while the length of epics can make them diluted and less unified. I think I still prefer epics to tragedies, but I can really see Aristotle's points. Interestingly though, of the epic poetry I've read, my favourites (e.g. Paradise Lost and the "Divine Comedy") were non-Greek; I think I enjoyed the plays of Aeschylus and Sophocles more than The Iliad. It will be interesting to see what I think of The Odyssey when I get to it on the reading list.