Hades was the King of the Underworld, the god of death and the dead. He presided over funeral rites and defended the right of the dead to due burial. Hades was also the god of the hidden wealth of the earth, from the fertile soil with nourished the seed-grain, to the mined wealth of gold, silver and other metals.
Hades was devoured by Kronos as soon as he was born, along with four of his siblings. Zeus later caused the Titan to disgorge them, and together they drove the Titan gods from heaven and locked them away in the pit of Tartaros. When the three victorious brothers then drew lots for the division of the cosmos, Hades received the third portion, the dark dismal realm of the underworld, as his domain.
Hades desired a bride and petitioned his brother Zeus to grant him one of his daughters. The god offered him Persephone, the daughter of Demeter. However, knowing that the goddess would resist the marriage, he assented to the forceful abduction of the girl. When Demeter learned of this, she was furious and caused a great dearth to fall upon the earth until her daughter was returned. Zeus was forced to concede lest mankind perish, and the girl was fetched forth from the underworld. However, since she had tasted of the pomegranate seed, she was forced to return to him for a portion of each year.
As Pluto, Hades is the God of Wealth (as plants and metal come from the underworld) and is hence highly regarded. Otherwise, as God of the Dead, he is feared, and Hades is not a good thing to speak. He will not help people if they give him sacrifices, though will occasionally answer people's curses. To pray to him, one must pound on the ground with rods or their hands. He was only worshiped as the God of the Underworld on Elis, though, as Pluto, he was more widly venerated, often with Dionysius, Demeter, and, of course, Persephone.
He is often referred to as the "Dark Zeus" or "Zeus of the Underworld", and is usually portrayed in a similar way as Zeus, except with dark hair, and a rather depressing look on his face. He, like the other male first generation Olympians, is seen as strong, muscular, and bearded.
When the male Gods drew lots for the Universe, Hades drew the Underworld. He never seems especially unhappy with his lot (it's Poseidon who rebels...not Hades, contrary to popular modern literature), and rarely appears on the surface. In fact, he was only seen to have appeared to flirt with Mintha, abduct Persephone, and be cured of an arrow wound inflicted by Hercules. However, if he did wish to appear, he has a helmet of invisibility, so no one would know. He is often one of the Olympains, but is rarely on Olympus (which, like the earth he shares jointly with his brothers). In many stories, even, he looses his chair to Demeter as he is so rarely there.
Hades is seen as very faithful to his wife (especially when considering Zeus and Poseidon), and is only known to have flirted with two nymphs, Leuce and Mintha, both of whom were swiftly changed into plants by Persephone. (In some stories, Leuce dies of a natural cause and Hades changes her, after her death, into a poplar)
Above all, though, Hades is a just, terrifying, and inexorable God. He is childless, and his wife often does not love him. (Even if she does, she is gone for half the year) He is "grim" and "fearsome", but he is neither evil, nor unjust. In fact, he may be the one just Greek God.