Hades is the Greek god of the Underworld where people go when they die (or so the ancient Greeks believed). In later times, many people preferred to call him Pluto, as the name "Hades" was considered to bring bad luck. He is also associated with wealth, hence the English word "plutocrat" meaning an extremely rich person.
Hades fought the Titans, together with his brothers, Zeus and Poseidon. Afterwards Zeus became ruler of the skies, Poseidon took the sea, and Hades became ruler of the Underworld.
The entrance to the Underworld is said to be along the River Styx in northern Greece. It is guarded by Cerberus the three-headed dog. In the Odyssey, the shades of the underworld are shown to be insubstantial - so that Odysseus is unable to hug his mother.
His queen is Persephone, daughter of Demeter, whom he abducted in a story that explains the seasons.
In the Odyssey we see that people worshiped Hades by sacrificing black sheep.
Later on, the Underworld itself became known as Hades.