Hades is the god of the dead and lord of the underworld, which is also called Hades after him. Elder brother of Zeus and Poseidon, he drew the realm of the dead as his share after the victory over the Titans. The Greeks avoided saying his name and preferred Pluto, "the rich one", because harvests and precious metals come from the earth: so he is also the god of underground riches. The Romans called him Pluto or Dis.
His character
Hades is neither a devil nor an evil god: he is stern and fair, applying the laws of death without exception and letting no one return to the living. He rarely leaves his kingdom and takes no interest in earthly affairs. He was not one of the twelve Olympians and had very few temples; to appease him, black animals were sacrificed and their blood poured into a pit.
The helmet of invisibility
During the war against the Titans, the Cyclopes gave him the kunée, a helmet that makes its wearer invisible. He lent it to others too: to Athena during the Trojan War and to Perseus so he could approach Medusa unseen.
Hades and Persephone
Hades's most famous myth is the abduction of Persephone, daughter of Demeter. The god rose from the earth in his chariot and carried the girl off to his kingdom to make her his bride. Demeter's despair made the earth barren until a deal was struck: having eaten a few pomegranate seeds, Persephone would spend part of the year with her husband and the rest with her mother. This myth explains the seasons.
The kingdom of the dead
Souls arrived guided by Hermes, crossed the river Acheron or the Styx in the boat of Charon, who was paid an obol, and passed Cerberus, the three-headed dog who let everyone in but no one out. Then they were judged by Minos, Rhadamanthys and Aeacus. See the Greek underworld.
Very few entered Hades's kingdom and came out alive: Heracles, who carried off Cerberus, Orpheus, who tried to bring Eurydice back to life, Theseus and Odysseus, who summoned the souls of the dead.
See also the Olympian gods and Cerberus and other monsters.