In Greek mythology, Tartarus was both a primordial deity that existed before the Olympians, as well as a name to describe a region of the Underworld. As a god, he was third in rank after Chaos and Gaea, preceding Eros.
As a place, it was far below than where Hades resided and it was used as the most horrible prison. Some accounts say that the distance between Tartarus and Hades was the same as between the earth and the heaven. Although the kingdom of Hades was the place of the dead, Tartarus was where ferocious monsters and horrible criminals were banished, or where the gods imprisoned their rivals after a war. The three judges of the Underworld, Rhadamanthus, Aeacus and Minos, decided who would go to the realm of Hades and who would be banished to Tartarus.
Moreover, Cronus, the king of the Titans, imprisoned the Cyclopes and the Hecatonchires in Tartarus, but Zeus released them in order to help him defeat the Titans. When the Titanomachy ended in favour of the Olympians, Zeus banished many of the Titans to Tartarus.
Tartarus is also called Tartaros.