OEDIPUS: Good son of Aegeus, gentle son,
only to the gods is it given not to age or die.
All else disrupts through all disposing time.
Earth ebbs in strength, the body ebbs in power.
Faith dies and faithlessness is born.
No constant friendship breathes
between man and man, or city and a city,
Soon or late, the sweet will sour,
the sour will sweet to love again.
Does fair weather hold between this Thebes and you?
Then one day shall ever teeming time
hatch nights on teeming days,
Wherein this pledge, this harmony, this hour
will break upon a spear,
slashed down for a useless word.
--Oedipus at Colonus
Though he lived well into his nineties, writing and producing plays from his twenties, only seven of Sophocles' 123 plays remain to us, a fact which seems appropriately tragic. In his time, Sophocles was regarded as the greatest that Greek tragedy had to offer, a dramatic Homer, and he won the first prize given to the best dramatist at the Athenian Dionysia twenty times. It is left to us to wonder whether the Greeks of Sophocles' time saw what we see in his plays--a persistent timelessness and depth of spirit--and whether it existed to the same degree in the 116 plays we have never seen.
Each of these seven is great in its own right: Ajax is the story of a warrior driven mad by jealousy, and mocked by the indifferent gods. Philoctetes is about a man who was abandoned by Odysseus on a desert en route to the Trojan War because his terrible snakebite made him irritating, but now Odysseus and Achilles' son Neoptolemus must return and convince Philoctetes to give them his bow, which never misses, so that they might use it to defeat the Trojans. The Woman of Trachis is about Deianeira, Heracles' wife, who struggles to love her husband in the face of his infidelity. Then there is Sophocles' take on Elektra, which to me is the least interesting of the seven, especially when placed next to Aeschylus' and Euripides' versions.
But the best plays here are the Oedipus plays, one of which I wrote about already (although Roche's translation improves on it significantly). It can't be said that Oedipus the King can't be understood without reading the other two in the "trilogy," because they were written at vastly different points of Sophocles' life and were never truly meant to create a unified whole, but I think they're vastly more interesting when read together
In Antigone, the first written, I think that you can see Sophocles' youth in Antigone's headstrongness and willingness to martyr herself. Antigone's brothers (Oedipus' sons) killed each other on the field of battle, one fighting for Thebes and the other against it, and her uncle Creon, the king, refuses to let the latter be buried properly. Knowing that it means she will be punished by death, Antigone buries him anyway, and commits suicide before Creon, having been enlightened by the prophet Teiresias, can come to pardon her.
On the other hand, Oedipus at Colonus was clearly written by a man reflecting upon his own age and mortality; it was one of the last plays that Sophocles wrote. It takes place between the events of Oedipus the King and Antigone, when Oedipus, having blinded himself, wanders with Antigone into the Athenian suburb of Colonus where they are welcomed by Theseus, the King. A prophecy has just told that whoever has Oedipus' favor will win the war for Thebes, and so King Creon and Oedipus' rebel son Polyneices have come to make amends, but Oedipus is too dignified to accede to their demands and for Theseus' warmth and kindness, Oedipus basically ensures Athens' dominance over Thebes in the decades to come.
Oedipus at Colonus is a pleasant reminder that Greek tragedy really isn't as rigid as Aristotle thought it should be, and that happy endings did exist. At the end of their lives, there is something appropriate about the way that Sophocles allows Oedipus--synonymous, for us and for Aristotle, with the spirit of tragedy--to be buried in dignity and love, and to be able to show the justice of a king in his final acts. Oedipus' words to Theseus, which I've copied above, affirm the inconstancy of the world, its chaos, entropy, and inevitable change, but without corrupting their basic truth, Oedipus is able to defy them as much as any human being is able. And though Sophocles could not have known this, the fact that Oedipus' story has survived for 2,500 years as an essential parable of downfall and redemption must temper his words as well.