"In Gehennom, only Moloch can hear you #pray." - Old Hacker aphorism...

Gehennom, taking its name from Gehenna (aka, the Hinnom valley outside Jerusalem, used in the scriptures as the metaphor for hell) is the "hell" in Nethack.

Generally speaking, it's not a really pleasant place to be. The legends from old ages tell that in previous versions of Nethack, the Hell was really Hell, and you couldn't survive without a ring of fire resistance. Now, I heard there was a shrewd shopkeeper, who previously had very powerful techno-magical skillz; He was called Izchak. He rewrote the Hell to be what is now known as Gehennom, a very different place indeed.

Gehennom in "vanilla" Nethack holds something more inconvinient in it than just fire and brimstone! In fact, it is not a place with fire and brimstone at all... It is a devilish maze. While most game players would find "fire and brimstone" version just an inconvinience, this one is really going to wreck their minds. In other words, perfect "Hell" material.

The Castle is the last level in the Dungeons of Doom; There are trapdoors in the Castle to get you in the Gehennom.

(Warning: Mild Spoilage Ahead)

Gehennom has 20 to 24 levels.

The bottom of the maze has the Inner Sanctum of Moloch, the place where the precious Amulet of Yendor is now kept. Getting there involves getting some artifacts from other places: The Bell of Opening, held by your Quest Nemesis in the Quest; The Candelabrum of Invocation, held by Vlad in his tower (a separate dungeon branch accessible through Gehennom - you can pray there); and, of course, the Book of the Dead, held by Rodney. You need to use those in correct place at correct time and correct order.

Rodney - such a cute and cuddly name for a big meanie called Wizard of Yendor - has his own Tower in Gehennom. Getting to the tower is somewhat of a challenge and the way construction of the tower may be perplexing for the uninitiated; Beware of the wizard's traps! Also, Rodney has this annoying tendency to not quit bugging you, and he certainly isn't thrilled about your idea to, ahem, aquire the Book.

Also of note are the domains of the demon lords. Lair of demon prince Asmodeus (who holds the gate to the deeper levels), the swampy home of Juiblex, the lair of Baalzebub, and the ghost town of Orcus.

Finally, the Valley of the Dead is the topmost level in Gehennom, and has a temple of Moloch.

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Sources: Well, mostly off the top of my head (not that I've ever been to Gehennom outside Wizard Mode, but I read the spoilers couple of times before =)... Dylan O'Donnell's Nethack 3.3 Spoilers helped me to Remember.

Ge'he'nnom

Gehennom is the Hebrew word for Hell.
The origin of the word is from the Bible, it was a place where worshipers of moloch made human sacrifices.

Guy - The Hebrew word for valley.
Henom - Local ruler who worshiped moloch and owned the valley.

Gehennom is not mentioned in the bible as a place where sinners go after they die, that was only mentioned in later writings.
Modern use for the word: "sh'e tisaref ba gehennom" meaning "burn in hell"

Gehennom is roughly the Jewish equivalent of a Christian Hell. According to the Talmud, the average soul after it leaves the body goes to Gehennom for 11 months (this is why it is customary for one who's parent died to say kaddish for 11 months after the shivah) where the soul is "purified" before it can enter Gan Eden roughly the Jewish equivalent of Heaven. A person who is excessively wicked goes to Gehennom for 12 months or, if he is very evil, can stay there forever or even have his soul destroyed. Contrariwise a person who is very righteous can avoid going to Gehennom entirely. While it is important to understand that there is no physical or tangible element to Gehennom I have heard the torture of Gehennom comparable to being suspended by ones feet and descending into boiling excrement.

The Talmud teaches that there are three people who go straight to Gehennom after dying. They are: the thief, the one who shames his friend in public, and the one who calls his friend by a bad name.

A Rabbi once explained the soul in a very helpful way. He said the soul is a clean white shirt. With every sin we stain that shirt differently. Gehennom is a laundromat. It's not easy, but we scrub ourselves clean because Gan Eden is an audience with God and a chance to experience pure joy.

I have heard one other analogy. The soul is a battery. When it cleaves to the body its potential expires. God is a power supply. Gan Eden and Gehennom is simply the act of recharging your soul. Now imagine the afterlife as a an amphitheater with God as the centrepiece. When we recharge we attach a straw to God through which we suck our potential. Depending on a person's relationship to God, a soul can cleave better to God in the afterlife. Gan Eden and Gehennom are simply how close you are to God and how fast you can reenergize you soul.

This coincides with a Chassidic parable of the afterlife. Envision a long table with the righteous on either side on the left and the wicked on the either side of the right. Placed before each person is his favourite food, the only catch is that no one can bend his arms. The righteous feed each other, the wicked learn to share or starve.

The reward for a good deed is the chance to do another.