Jahannam is the Muslim version of the Christian Hell or place of punishment.
In general terms the Christian Bible is rather vague about Hell whereas the Muslim text is painfully, dare one say soul-deadeningly, full of horrific detail. The litany of torments features a host of extremely painful punishments from which there is neither surcease nor deliverance. Compared to Jahannam, the Hell of the Bible is almost a trip to a day spa.
The term Jahannam is based on the Hebrew Gehinnom.
Conditions
The conditions within Jahannam are far from pleasant. It was initially a fiery red but the flames were fanned until they increased and the fires burned white hot. The heat was increased yet again until the place is totally black, giving forth no light at all. This heat was produced by 3,000 years of continuous stoking of the flames. The most extreme heat of flame upon the Earth is but 1/70 that of Jahannam. In moden terms it sounds suprisingly like a black hole.
Size matters
The size of Jahannam is adequate to contain all the persons deserving of its torments, the tormenters themselves, as well as various other features. If a stone were cast into Jahannam it would take 70 years to reach the bottom. From the center of Jahannam it would be a journey on foot of 40 years to reach the walls. Entrance is through one of seven gates. There are no exits.
The place that keeps on giving
One of the features of Jahannam are trees which bear a bitter and thorny fruit which is the sole food of the inhabitants. The fruit gives no refreshment, satisfies no hunger, but instead boils within like molten metal. The drink will be the blood and other vile substances and excrements emitted by those being punished. For variety in the drink offerings there is also boiling water available upon request.
The least of the punishments available in Jahannam are to be fitted with shoes of flame. The intense heat of these shoes causes the blood and brains of the hapless wearer to boil. This is as good as it gets in Jahannam, as every other punishment is even worse.
Avenging angels, 19 in number, are tasked with inflicting punishments upon those contained in Jahannam. Some of their torments feature pouring boiling water upon various parts, shearing off the lips with shears of flame and sundry other amusements for the suffering of the inhabitants.
Within Jahannam are scorpions the size of donkeys and strange camel-like snakes which bite. The stings and bites burn as flame and the bitten cannot die but must continually endure the torments from these creatures. Also for your torment is the ever popular being beaten with hot rods of iron. These rods or clubs are of sufficient mass as to be unmovable by all the inhabitants of the Earth, should they be siezed by a sudden desire to lift one. All this flailing by rods will of course cause skin damage, but rest assured; the skin is regenerated just in time for your next flogging.
Down, down, down to a burning ring of fire
Like Dante's Inferno, Jahannam has levels; whereas the Inferno contained 9 levels Jahannam has only 7. The highest level is that for unbelieving Muslims. The second level is that of the Christians. The third level is reserved for the Jews. Other levels contain specific groups similar to the top 3 already mentioned.
So, how does one stay out of Jahannam?
To avoid this terrible fate and find oneself in Jannah, the Muslim Heaven, is to live a life of piety and obedience. Failure to believe the Prophet, failure to strictly adhere to his teachings and directives lead one inevitably to eternity in Jahannam.
Sources:
http://www.inter-islam.org/faith/hell.htm
http://truelife200vi.files.wordpress.com/2007/09/jahannam.pdf
http://www.pantheon.org/articles/j/jahannam.html
http://www.geocities.com/thanabhavan/qiyama