Hades is the Greek god of the dead. He was the son of Cronus and Rhea, thus brother of Zeus, Poseidon, Hera, Hestia and Demeter. Hades constantly lived under the earth in his domain – Tartarus, or Underworld, or Hell. Hades, the name means 'invisible', was not usually named out loud for fear of the god; instead euphemisms were used to name him, and one of such names was Pluto, which means 'rich', with the hint for vast richness of the earth.
See: Caravaggio Jupiter, Neptune and Pluto.