Tragedy has a characteristic structure in which scenes of dialogue alternate with choral songs. This arrangement allows the chorus to comment in its song in a general way on what has been said and/or done in the preceding scene. Most tragedies begin with an opening scene of expository dialogue or monologue called a prologue.
After the prologue the chorus marches into the orchestra chanting the parodos. Then follows a scene of dialogue called an episode, which in turn is followed by the first stasimon. The alternation of episode and stasimon continues until the last stasimon, after which there is a final scene of dialogue called an exodos 'exit' scene'. The exodos is in general a scene of dialogue, but, as in the case of episodes, sometimes songs are included, especially in the form of a kommos.
Here is the structure of a typical tragedy (some tragedies have one more or one less episode and stasimon):
Source: http://ablemedia.com/ctcweb/netshots/tragedy.htm