The origins of Greek theater are shrouded in mystery, but it likely began as a form of religious worship. In Athens during the sixth century B.C.E., theater was a part of the rituals honoring the god Dionysus. Weeklong festivals every spring would include dances, chants, public drinking, and orgies, as well as religious chants. Those chants slowly evolved into formalized works of drama.
Greek plays were usually performed in outdoor amphitheaters, adapted from public parks and city squares. The main stage was round, with raised seats arranged in a semi-circle, rising slowly away from it. A skene (or tent) stood behind the stage, allowing actors to enter and leave the stage; they could also appear on the roof of the skene when portraying gods or similar "elevated" characters.
Greek plays usually used three actors, along with a chorus, which served as observers and commentators on the drama. The actors would wear masks that covered their entire faces, allowing them to take on multiple roles in the drama in plays with more than three characters. The masks had exaggerated features--allowing distant audience members to better understand the characters' emotions--and enlarged holes around the mouths to let the actors speak clearly.
Ancient Greek plays were separated into two basic genres--tragedy and comedy. The former dealt with human suffering and the folly of life, with the characters coming to a bad end (often because of their own mistakes). The latter dealt with the joys and happy occasions in life. Tragedy emerged first--sometime in the sixth century B.C.E.--with comedy arising in the middle of the fifth century. The three major tragedians were Aeschylus, Sophocles, and Euripides; the major comic playwright was Aristophanes. Many of their works are now lost, but those that survive rank among the glories of ancient literature. The two genres supplied the comedy and tragedy masks, which today serve as a universal symbol for theater.
Greek plays began with a prologue, in which one character would speak to the audience and set up the drama. That was followed by a parados, in which the chorus appeared and chanted or danced. The first act would follow, then a stasimon, in which the actors departed and the chorus returned for more chanting or singing. Further acts would take place, each followed by a stasimon, until the finale or exodos. During the exodus, the chorus sang a last song, intended to convey the primary "lesson" of the play.