Nemesis 

Nemesis was the goddess of divine indignation and retribution, who punished excessive pride, evil deeds, undeserved happiness or good fortune, and the absence of moderation. She was the personification of the resentment aroused in both gods and mortals by those who committed crimes with impunity, or who enjoyed undeserved luck.

True to her name, which variously may be translated as 'she who distributes or deals out'; 'due enactment'; or 'divine vengeance', Nemesis was a feared and revered goddess. With a discriminating eye she directed human affairs in such a way as to
maintain equilibrium on earth. Happiness and unhappiness were measured out by her, with firm care being taken that happiness was not too frequent or too excessive. If this happened, Nemesis could bring about abrupt and catastrophic losses and suffering.

As one who kept extravagant favors by Tyche (Luck, Fortune) in check, Nemesis was regarded as an avenging or punishing divinity. Tyche was often irresponsible in handing out Luck and Fortune, indiscriminately heaping gifts from her horn of plenty, or depriving others of what they had.

But woe be to the individual favored by Tyche who failed to give proper dues to the gods, became too full of himself and boasted of his abundant riches, or refused to improve the lot of his fellow humans by sharing his luck! Indignant Nemesis would step in and snap the fool back to reality, in short order humiliating him and causing his downfall.

Along with Dike and Themis, wise goddesses of Justice, Nemesis was one of the assistants of Zeus, the king of the Olympian gods who was regarded as the founder of law and order. Her home was at Attic Rhamnus, site of a magnificent sanctuary dedicated to the feared goddess of divine vengeance.

About sixty stades from Marathon as you go along the road by the sea to Oropus stands Rhamnus. The dwelling houses are on the coast, but a little way inland is a sanctuary of Nemesis, the most implacable deity to men of violence. It is thought that the wrath of this goddess fell also upon the foreigners who landed at Marathon. For thinking in their pride that nothing stood in the way of their taking Athens, they were bringing a piece of Parian marble to make a trophy, convinced that their task was already finished.

Of this marble Pheidias made a statue of Nemesis, and on the head of the goddess is a crown with deer and small images of Victory. In her left hand she holds an apple branch, in her right hand a cup on which are wrought Aethiopians. As to the Aethiopians, I could hazard no guess myself, nor could I accept the statement of those who are convinced that the Aethiopians have been carved upon the cup because of the river Ocean. For the Aethiopians, they say, dwell near it, and Ocean is the father of Nemesis.

Beautiful Nemesis initially was portrayed without wings, but in later descriptions she appeared as a winged goddess. In her left hand she held an apple-branch, rein, lash, sword, or balance. Her symbols and attributes were like those of Tyche: a wheel and a
ship's rudder.

Her parents were said to be either Nyx (Night) alone without a father, or the Titans Oceanus and Tethys:

"Also deadly Nyx bare Nemesis to afflict mortal men."
-Hesiod, Theogony 223

“Alexandros [the Great] was hunting on Mount Pagos [near Smyrna], and that after the hunt was over he came to a sanctuary of the Nemeseis, and found there a spring and a plane-tree in front of the sanctuary, growing over the water. While he slept under the plane-tree it is said that the Nemeses appeared and bade him found a city there and remove into it the Smyranians from the old city … So they migrated of their own free will,
and believe in two Nemeses instead of one, saying their mother is Nyx, while the Athenians say that the father of the goddess in Rhamnos is Okeanos.” -Pausanias, Description of Greece 7.5.3

Nemesis was also known as Adrasteia, which means 'inescapable', or 'Tracing Goddess'. You could say that Nemesis/Adrasteia was the ancient Greeks' conscience,
for the goddess of retribution personified moral reverence for the natural order of things and provided a deterrence to wrongful action.

She was also called Rhamnusia or Rhamnusis, in honor of her sanctuary in Rhamnos.

Nobody wanted to be hounded by Nemesis, and even to this day her name means:

1.A source of harm or ruin: "Uncritical trust is my nemesis."
2.Retributive justice in its execution or outcome: "To follow the proposed course of action
is to invite nemesis."
3.An opponent that cannot be beaten or overcome.
4.One that inflicts retribution or vengeance.

Thus, you'll often read or hear quotes such as:

"This is that ancient doctrine of nemesis who keeps watch in the universe, and lets no offense go unchastised." --Emerson.

A famous example of the retribution of Nemesis is the story of Narcissus. This man was the beautiful son of the River Cephissus and the nymph Liriope. He was so handsome that all women who beheld him at once fell in love with him. The vain Narcissus, however, only had eyes for himself (you could say he suffered from "I" strain...) and rebuffed all admirers.

One such admirer was the nymph Echo, who saw Narcissus and at once fell in love with him. But the beautiful youth couldn't be bothered with the smitten one, who slowly pined away, leaving just the echo of her voice.

Nemesis saw this and condemned the vain Narcissus to spend the rest of his days admiring his own reflection in the waters of a pool. Eventually Narcissus died and was transformed into the flower that bears his name.

Nemesis is considered by some to be the mother of Helen and the twins called the Dioscuri. It's said that Zeus once fell in love with Nemesis (she had quite a bit of Aphrodite's beauty, and some said she was just as gorgeous) and relentlessly pursued
her on land and sea. Leery of his intentions, Nemesis avoided Zeus by constantly changing forms, finally transforming into a goose. Not to be outdone, Zeus in turn took the form of a swan, and from the egg she laid came Helen, the ultimate cause of the famous Trojan War.

But some say that Helen was a daughter of Nemesis and Zeus; for that she, flying from the arms of Zeus, changed herself into a goose, but Zeus in his turn took the likeness of a swan and so enjoyed her; and as the fruit of their loves she laid an egg, and a certain shepherd found it in the groves and brought and gave it to Leda; and she put it in a chest and kept it; and when Helen was hatched in due time, Leda brought her up as her own
daughter. 

Harry Thurston Peck, in Harpers Dictionary of Classical Antiquities (1898), tells us that Nemesis was:

A post-Homeric personification of the moral indignation felt at all derangements of the natural equilibrium of things, whether by extraordinarily good fortune or by the arrogance usually attendant thereon. According to Hesiod (Theog. 223) she is the daughter of Night (Nyx), and with Aidos, the goddess of Modesty, left the earth on the advent of the Iron Age. A legend makes her to have been by Zeus the mother of Helen and the Dioscuri. As goddess of due proportion she hates every transgression of the bounds of moderation, and restores the proper and normal order of things. As, in doing this, she punishes wanton boastfulness, she is a divinity of chastisement and vengeance.

She enjoyed special honor in the Attic district of Rhamnus (where she was believed to be the daughter of Oceanus), and is often called the Rhamnusian goddess. Her statue there (of which fragments were found in 1890) was said to have been executed by Phidias out of a block of Parian marble which the Persians had brought with them in presumptuous confidence to Marathon, to erect a trophy of victory there. She was also called Adrasteia, that name appropriate only to the Phrygian Rhea-Cybelé, being interpreted as a Greek word with the meaning, “She whom none can escape.” She was also worshipped at Rome, especially by victorious generals, and was represented as a meditative, thoughtful maiden with the attributes of proportion and control (a measuring-rod, a bridle, and a yoke), of punishment (a sword and scourge), and of swiftness (wings, a wheel, and a chariot drawn by griffins).