Gehenna is the name for Judaism's equivalent of the Christian Hell. However, Gehenna is a place of purging, the idea being that one's soul should emerge from it clean "like a firebrand pulled from the flames". Some say that everyone, even tzadikim, visit Gehenna for a period of 30 days before continuing to Heaven. The most interesting description of Ghenna that I have ever heard is that you and everyone who knows you sits down and watches a movie of your life from your POV and including your thoughts at those times. And you can't leave.
Geographically speaking, Gehenna is indeed a place. It is the Gei Hinom Valley just outside Jerusalem's Old City. The Damascus Gate is the closest entrance to the valley. Gei Hinom, a loose translation for Ben Hinom, or "son of Hinom", was a place where the local pagan tribes conducted their rituals which included, but were not limited to, child sacrifice, orgies, beastiality, and so forth. Blood ran through the area in rivers, provoking the new connotation.
The Gei Hinom Valley was revived by a mention in "Life of Brian" when they refer to the Valley of the Cheesemakers, or the Trippolean Valley.