Hades was the ruler of the Greek Underworld (which itself is sometimes confusingly referred to as "Hades" also). In mythology, he was the brother of Zeus and Poseidon, and therefore from the first generation of Olympian gods. And the manner in which Hades became the lord of the shadowy Underworld is worth mentioning.
According to the myth, the three sons of Kronos and Rhea - Zeus, Poseidon, and Hades - each drew lots to determine which part of the world they would rule. Zeus and Poseidon received the sky and the seas respectively, and Hades was therefore left with the Underworld.
Hades ruled the realm of the dead alone. That is, until he became enamored of the beautiful goddess Persephone. The legend of how Persephone became the bride of Hades and the Queen of the underworld is told in the Hymn to Demeter. Although the relationship between Hades and Persephone appears to have begun badly - Hades abducted his future bride and whisked her away in a chariot - their union was peaceful (unlike the marriage of his brother Zeus...).
Hades was identified with Dis (Dis Pater) in Roman mythology.
Hades and the Underworld
Since Hades was the ruler of the Underworld, it makes sense to note one of the key features of this region - its myriad rivers. These rivers each had names and symbolic meanings: