Greek tragedy was structured by alternating dialogue with song, usually in a repeating pattern. The Chorus, which sung the songs (and danced as well) would then comment on the action that had just happened in song, as well as sometimes predicting what was to happen next.
Most extant tragedies begin with a prologue, a long monologue introducing the situation of the play and setting it in context.
After the prologue has finished, the Chorus enter the orchestra, chanting the parados (sometimes also spelled parodos) - their opening song.
Next comes a scene of dialogue, which are usually referred to as episodes. After that, the main characters leave the stage, and the Chorus sing another song, commenting on the action, called a stasimon. Often the stasimons will comment on the scene as well as setting it into its mythological framework.
Alternating episodes and stasimons continues until the play reaches it conclusion: the exodos, a final scene of dialogue (sometimes it might also contain songs). The Chorus then sing a final song as they exit the orchestra.