Hades (Άδης or Ἀΐδας; from Greek ᾍδης, Hadēs, originally Ἅιδης, Haidēs or Άΐδης, Aidēs, meaning “the unseen”) refers both to the ancient Greek underworld, the abode of Hades, and to the god of the underworld. Hades in Homer referred just to the god; the genitive ᾍδου, Haidou, was an elision to denote locality: “[the house/dominion] of Hades”. Eventually, the nominative, too, came to designate the abode of the dead.
In Greek mythology, Hades is the oldest male child of Cronus and Rhea. According to myth, he and his brothers Zeus and Poseidon defeated the Titans and claimed rulership over the cosmos, ruling the underworld, air, and sea, respectively; the solid earth, long the province of Gaia, was available to all three concurrently.
Hades was also called “Plouton” (Greek: Πλούτων meaning “Rich One”), a name which the Romans latinized as Pluto. The Romans would associate Hades/Pluto with their own chthonic gods; Dis Pater and Orcus. The corresponding Etruscan god was Aita.
Symbols associated with him are the Helm of Darkness and the three-headed dog, Cerberus. The term hades in Christian theology (and in New Testament Greek) is parallel to Hebrew sheol, and refers to the abode of the dead. The Christian concept of hell is more akin to (and communicated by) the Greek concept of Tartarus, a deep, gloomy part of hades used as a dungeon of torment and suffering.