In Greek mythology Hades reigned over the dead as the ruler of the Underworld. Though one of the major Olympian gods, he is somewhat separated from the rest of the gods and goddesses because of his unique position. Rather than reside on Mount Olympus with Zeus and others, Hades is forced to hold over the Underworld - the land of the dead where those who have passed away become his subjects.
Throughout Greek mythology Hades been firmly planted in the role of death, this wasn't necessarily always the case. In fact, with a little better luck he could have had a pretty sweet gig ruling over the sky, sea, or land. After overtaking the previously ruling Titans, Hades and his brothers Zeus and Poseidon decided to divide control of the cosmos into three parts - the sky and Heavens, the sea, and the Underworld. All three, along with the other gods and goddesses would share dominion over the land and those who lived upon it. To make things fair, the brothers drew lots to see who would command which. Best pick got the Heavens, second got the sea, and the worst draw (which nobody really wanted) was the Underworld. Zeus got the Heavens, Poseidon the sea, and Hades literally got to live in a hellhole.
Comparing the Underworld (which is often also called "Hades") to the modern vision of Hell is not totally accurate. For most major modern religions, the concept of Hell is a (usually fiery) land of fear and agony where the wicked go to spend the rest of eternity paying for their earthly sins. In Greek mythology Hades (the Underworld, not the god) was a place where human souls would go often regardless of what they had done in their corporeal lifetimes. The only real alternative was being made an immortal by the gods, where one would get to live in Zeus's realm as lesser than the gods but higher than living humans. Being given immortality was incredibly rare and only the great heroes of Greek mythology could even sniff at this possibility. Nobody could count on it happening as a guarantee.
That said, the Underworld wasn't
exactly the "place to be". The other gods greatly preferred living on
any of the other planes of the cosmos leaving Hades on his own for the
most part. All alone and unable to find a mate among the goddesses, he
petitioned his brother Zeus to give him a bride to live in the
Underworld with him. Zeus knew that none of the goddesses would want to
live in the Underworld so he devised a plan to kidnap his own daughter, the
Spring goddess Persephone, and force her to become Queen of
the Underworld. The news of her daughter's forced marriage did not
please Demeter, goddess of harvests, who threatened to ruin all the
crops of the earth if her daughter was not returned. This left Zeus
with a difficult decision. He could not deny his brother of a wife but
at the same time he could not let all of humanity starve to death, so he
came up with a compromise. Every year in the Spring season Persephone
was allowed to leave the Underworld and return to the earth where she
would herald the end of Winter and the growth of new plants. When
Winter began again Persephone would return to the Underworld, causing
plant life to die off until her return.
One of the lesser known aspects among the myths of Hades was that the god of death was also considered a god of riches. In Greek mythology Hades ruled over the buried wealth of the earth, including gold, silver, and other minerals as well as the rich, fertile soil that helped grow great harvests. It was said that this King of the afterlife was exceptionally concerned with the loyalty of his subjects, most of whom were the dead who lived with him in the Underworld. He employed the three-headed dog Cerberus to guard the gates in and out of his realm to ensure the living could not enter and the dead could not leave.