Furies (Latin Furiae) were the female spirits of justice and vengence. They were said to have been born from the blood of Uranus that fell on Gaia when Cronos castrated him - or according to another version they were the offspring of Nyx.
There are usually three of them: Alecto, Megaera and Tisiphone. They had the effect of sending their victims mad, hence their name, from furor, 'raving', 'madness'.
Cicero and others identified them with a somewhat obscure Roman goddess called Furina or Furrina, who was believed to have had a grove on the banks of the river Tiber in Rome and to have been a kind of goddess of the spring. The Greek name for the Furies was Erinyes.