There was actually a good episode on the History Channel called...I believe either... Layers of the Underworld, Clash of the Gods, or Gates of Hell. It explained, in detail, the several layers of Hades and their purposes.
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-Hades (domos Aidaou) was the land ruled by Hades. Every soul eventually found it's way to Hades and was then sent to their eternal home, good or bad.
-Tartarus was the realm originally meant to hold the Titans, but later the place for all the damned souls meant to be tortured for eternity.
-Erebus was the personifaication of darkness; sometimes seen as a place and sometimes a god. From Erebus was birthed Nxy, Styx, Tanatos, and several others. It was the region souls had to walk through immediately after death. The same realm mention in the myth of Orpheus.
-Asphodel Meadows was the place for souls who lived average lives, having done an equal amount of good and bad. The place was filled with Asphodel flowers which, apparently, was a favorite food amongst the dead. Sometimes it's described as a type of limbo. No pain, but no pleasure. No fear, but no joy. No hate, but no love. No cold, but no heat...etc.
-Styx, the river of hate, was the boundary between Earth and Hades; it circled the realm of the dead nine times. Phlegethon, Acheron, and Cocytus are all said to meet in the center of Hades in a great pool also called Styx. The river Styx is what the gods swore by. If a god swore on the river Styx and broke their word Iris would fetch a cup of water from the river and the god would have to drink it and go one year without ambrosia. The water was pretty toxic and without the ambrosia they lost, I believe, their god-powers for a year or either the ability to speak. Achilles was dipped in the Styx as an infant and was granted immortality (except on his heel which his mother failed to soak).
-Acheron was the river of woe.
-Cocytus was the river of lamentation.
-Phlegethon was the river of fire.
-Lethe was the river of forgetfulness.
-The Elysian Fields was a place meant for heroes and those chosen by the gods to be rewarded for specific deeds. It was a place of enjoyment. Whatever pleasures they enjoyed in life they were able to enjoy in death.