The funny thing about Christian (and Muslim) hell is that its all a big mistake, an amusing misunderstanding of what is essentially a Jewish literary metaphor. Gehenna (in Arabic, Jahannan) is an actual place, Geh (valley) of the sons of Hinnom. Gehenna was a locale on the outskirts of old Jerusalem. It's where the worshippers of the Canaanite God Moloch sacrificed their children and burnt their remains as an offering to their god. So when the Bible mentions Gehenna as a place of fire and torment, they were describing an actual locale, the worst place imaginable, like Chernobyl or Philadelphia. It's a trope, a metaphor, a literary conceit, understood as such by sophisticated Jewish theologians.
Unfortunately, it was taken literally by credulous hick Christians. They thought Gehenna was an actual place in the afterlife, and proceeded to base much of their theology on the misunderstanding. What is Christianity without the threat of hellfire and damnation? A few hundred years later, the Muslims borrowed the concept from the Christians and elaborated upon it, even claimed they thought of it first.
Oops.
Judaism actually does have a Hell. It's called Sheol and is similar to Christian Purgatory, a place of chastisement where imperfect souls are sent to cleanse them in preparation for the World to Come. Stays there last no more than a year; we're so certain of it we stop prayers for the dead after eleven months.
So what happens to the irredeemably evil in Judaism? They simply cease to exist. The real threat in Judaism is to not have your name entered in the Book of Life; essentially, to be forgotten by God.
