In classic Greek mythology, below Heaven, Earth, and Pontus is Tartarus, or Tartaros (Greek ????????, deep place). It is either a deep, gloomy place, a pit or abyss used as a dungeon of torment and suffering that resides within Hades or the entire underworld with Tartarus being the hellish component. In the Gorgias, Plato (c. 400 BC) wrote that souls were judged after death and those who received punishment were sent to Tartarus. As a place of punishment, it can be considered a hell. The classic Hades, on the other hand, is more similar to Old Testament sheol.Like other primal entities (such as the earth and time), Tartarus is also a primordial force or deity.
Tartarus in Greek Mythology
In Greek mythology, Tartarus is both a deity and a place in the underworld even lower than Hades. In ancient orphic sources and in the mystery schools Tartaros is also the unbounded first-existing "thing" from which the Light and the cosmos is born.In Hesiod's Theogony, c. 700 BC, the deity Tartarus
Source:
http://en.wikipedia.org/wiki/Tartarus