In Greek mythology the Erinyes (the Furies) were female personifications of vengeance. They were usually said to have been born from the blood of Uranus that fell upon Gaea when Cronos castrated him; ie, they were cthonic (Earth) deities. According to a variant account, they were born from Nyx. Their number is usually left indeterminate, though Virgil, probably working from an Alexandrian source, recognized three; Alecto ("unceasing"), Megaera ("grudging"), and Tisiphone ("avenging murder"). The heads of the Furies were wreathed with serpents, and their whole appearance was terrific and appalling.
The Furies generally stood for the rightness of things within the standard order; for example, Heraclitus declared that if Helios decided to change the course of the Sun through the sky, they would prevent him from doing so. But for the most part they were understood as the persecutors of mortal men and women who broke "natural" laws. In particular, those who broke ties of kinship through patricide, murdering a brother, or other such familial killings brought special attention from the Furies. It was believed in early epochs that human beings might not have the right to punish such crimes, instead leaving the matter to the dead man's Furies to exact retribution. The goddess Nemesis filled a similar role. When not stalking victims on Earth the Furies were thought to dwell in Tartarus, the Greek version of hell, where they applied their tortures to the damned souls there.
The Furies typically had the effect of driving their victims insane, hence their Latin name furor.