When we talk about Aristotle's theory of art, we are actually discussing his treatments of "those things pertaining to the Muses" or, in practice, music, poetry, sculpture and painting. These are discussed in several of Aristotle's extant works, Poetics, Politics, Nicomachean Ethics, and Rhetoric.
All arts, for Aristotle (though music is somewhat of an exception), are forms of imitation or mimesis. Visual arts imitate by means of paint or stone and verbal arts by means of words. We derive natural pleasure from imitation. The arts can benefit the polis because they function to train the emotions. In the case of tragedy, for example:
...through pity and fear effecting the proper purgation of such emotions.