Hades was the Greek god of the Underworld. His brothers were Zeus and Poseidon. His father Cronus was the lord of the Titans and his mother was Rhea, the Mother of Gods. He was the eldest son to his parents, and like all his siblings other than Zeus, he was devoured by his father until Cronus was forced to regurgitate.
As the Ruler of the Dead
He received the Underworld as his domain as Zeus received the leadership of Mount Olympus and rules over the dead. Hades has assistants who obey him without a question.
Although Hades is free to leave Underworld and walk among mortal men, he rarely does so. He has little interest in the affairs of world above. He is obsessed with making sure no-one who enters his realm leaves.
Hades is mostly indifferent about his subjects as long as they aren't attempting to escape, when someone does escape or otherwise cheats death, his fury is overwhelming and he will go to any lengths to set things right.
In order to get to the Underworld, it was said that you have to go to Charon, and pay him drachmas.
Hades and Persephone
Persephone was beautiful daughter of Zeus and Demeter, and Hades fell in love with her. Demeter wouldn't approve their union, but Zeus gave his permission that Hades could abduct Persephone to Underworld.
Demeter replied with unleashing a terrible curse of famine upon mankind that threatened to deprive gods of their source of gifts and worship if Persephone was not returned to her.
Gods pleaded Hades to let go of his captive, to which he agreed but not before Persephone was to eat a pomegranate seed.
When Persephone confronted her mother, it became apparent that once you eat food that is meant for the dead, you can never fully leave the land of the dead. Thus a marriage was arranged between Hades and Persephone, and she was to spend one third of a year in the Underworld.