Hades and Persephone in the underworld
Hades was the god of the dead, who ruled the place where dead people went after they died. He is a rather shadowy figure in more ways than one, spooky, and the Greeks preferred not to talk about him too much. Generally people who had good intentions did not sacrifice to Hades either. When they did, instead of burning the fat and the bones so the smoke would go up to the sky, instead they poured blood into pits or ditches dug into the ground (as in the part of Homer's Odyssey where Odysseus visits the Underworld).
Hades was thought of as the brother of Zeus and Poseidon, and therefore also the brother of Demeter and Hera.
Like Poseidon, Hades does not appear in very many Greek myths. The best-known of the myths he is in are those of Persephone and Orpheus.
People sacrificed to Hades when they wanted something bad to happen, like if they were trying to get revenge on an enemy.
To find out more about Hades and the underworld, check out these books from Amazon or from your local library:
Persephone and the Pomegranate: A Myth from Greece, by Kris Waldherr (1993). For kids. Not cheap, but beautifully illustrated.
Myth-O-Mania: Have a Hot Time, Hades! by Kate McMullan (2002). For kids - a "cool" retelling of myths from a different point of view.
D'aulaire's Book of Greek Myths, by Edgar and Ingri D'Aulaire.
Greek Religion, by Walter Burkert (reprinted 1987). By a leading expert, for adults. He has sections on each of the Greek gods, and discusses their deeper meanings, and their function in Greek society.
Aspects of Death in Early Greek Art and Poetry, by Emily Vermeule (1979). She's an expert on early Greece, and this book goes into detail about what the Greeks thought happened to people after they died. For adults.