Greek plays start from the idea of a chorus or “dance troupe,” composed of 12 (or earlier probably 15) young Athenian citizen men, who have been practicing four to six months in order to wow their audience of friends, neighbors, and family members. Of course, there were also the professional actors whose craft was their livelihood; but the local men’s performance was what many folks really came to see, and probably something that many remembered most vividly about the event. The chorus’ entrance is the official start of the play, and is called the Parodos. It is typically a big, flashy number in which the chorus enters en masse. They remain onstage until the very end of the play. When they leave, the play is over. Out of all the plays we have, in only one (Sophocles’ Ajax) does the chorus apparently leave the stage for a brief interlude–the moments in which Ajax commits suicide alone on the strand.
In a way, you can think of a Greek tragedy as a series of dance and singing numbers interrupted by scenes with actors speaking with one another, and with the chorus itself (or a chief chorus member, called the coryphaios or “chorus leader”) in iambic trimeter, something like the Greek equivalent of Shakespeare’s “blank verse” iambic pentameter. The Greek word for “actor”, hypokrites, or “one who answers back,” contains a vestige of the origins of theater in a chorus, in that it means something like, “someone who answers questions posed to him/her by the chorus.” Suggestively, the Greek word for “produce a play” is choregein, “to lead a chorus.”
Contrary to what you sometimes hear people say, the chorus is not just there to decorate the stage, or to “comment on the action” of the play. The chorus is intrinsic to the action–in fact, a chorus always plays a particular character in the play. Not only are they physically crucial to the performance, but a Greek chorus is not an undifferentiated set of individuals – they are a group character. Some Greek choruses portray the authoritative “old men of the town,” – see, for instance, Sophocles’
Antigone, Euripides’ Heracles and many others. In other plays, you find outsiders and relatively “powerless” people in the chorus: slave women (Aeschylus’ Libation Bearers, Euripides’ Iphigenia in Tauris and Helen), foreigners (Aeschylus’ Persians, and of course Euripides’ Bacchae), women of the town generally (Euripides’ Medea, Sophocles’ Women of Trachis), and other “outsiders.” For an excellent discussion of the identity and authority (or non-authoritative nature) of the Greek choruses, see Helene Foley’s now-classic article, “Choral Identity in Greek Tragedy,” in Classical Philology Vol. 98, No. 1 (January 2003), pp. 1-30.
A chorus is not a neutral commentator on actions that have nothing to do with it; it may sometimes say things that sound lofty, but it is speaking from a human (or occasionally divine) perspective, and cannot be taken as expressing the opinions of its playwright or author. Any chorus, in character, has opinions about what is going on, and suggestions to make — but of course, they are not always correct – sometimes they lead spectators on a wild goose chase, and sometimes they come to a standstill when they don’t know what to do. The chorus drives the action of the play, and is a character itself, sometimes (as in Aeschylus’ Libation Bearers) even intentionally tricking and throwing the bad guy off track so that the hero can get the best of him or her. In Greek tragedies and comedies, the chorus is definitely the thing.