This is the Muhammadan equivalent to Hell in Christianity. This word and concept did not exist in the Arabic language prior to the Quran since it was derived from the HebrewGehinnom, which was the name of a valley near Jerusalem where human sacrifices were made by the pagans.
According to the Quran only Allah knows who will go to Jahannam and who will go to Jannah (Paradise). Those who ignored, or only pretended to believe in Allah will remain in Jahannam after Qiyamah (Judgment Day).
Jahannam has seven gates (Quran 59. 71; 15. 43) and different levels, the lowest being the tree Zaqqum and a cauldron of pitch and fire. Punishments are in accord to the gravity of sins - a theme much elaborated on by later commentators.
37: 62Is that the better entertainment or the Tree of Zaqqum?
63For We have truly made it (as) a trial for the wrongdoers.
64For it is a tree that springs out of the bottom of Hell Fire:
65The shoots of its fruit-stalks are like the heads of devils:
The Quran is not clear if the punishments exacted on Muhammadan sinners last forever, and Muhammad's statement,
"The condemned will be cast into fire, to dwell there so long as the heavens and earth shall last, unless Allah wills otherwise," does not make it clear as whether Allah ever would "will otherwise."
*** Allah - if Allah is the same as the God of Israel - could not have told Gabriel about Jahannam since it was never part of His creation.
the concept of Jahannam/Hell is not mentioned or discussed in the Torah. If God had created such a place as Jahannam,He would most certainly have threatened the Israeliteswith such horrible punishments if and when they disobeyed or broke His rules.
No where in the Torah is such a threat used by God. The concept of Gehinnom/Jahannam/Hell is a post Biblical one which Muhammad and his Quran - as usual - plagiarised and imbellished with his own visions to scare his followers into submission.
The descriptions of Hell that he pirated are found in the Midrash of the Ascension of Moses called Gedullat Mosheh, of which there is a manuscript of an Arabic translation in the library of Berlin.
All these traditions and influences were available to Muhammad since he was surrounded by them and was able to pick and choose what suited his purposes at will.
II Kings 23: 10 "And he defiled the Tophet, which is in the valley ofBen Hinnom, that no man might make his son or his daughter to pass through the fire to Molech.
Jeremiah 7: 31 "And they have built the high places of Tophet, which is in the valley of Ben-Hinnom,to burn their sons and their daughters in the fire; which I did not command them, nor did it come into my heart.
32. Therefore, behold, the days come, says the Lord, that it shall no more be called Tophet, nor the valley of Ben-Hinnom, but the Valley of Slaughter; for they shall bury in Tophet, because there is no place elsewhere.
II Chronicles 28: 1. "Ahaz was twenty years old when he began to reign, and he reigned sixteen years in Jerusalem; but he did not do that which was right in the sight of the Lord, like David his father;
2. For he walked in the ways of the kings of Israel, and made also molten images for the Baalim.
3. And he burned incense in the Valley of the Son of Hinnom, and burned his children in the fire, according to the abominations of the nations whom the Lord had cast out before the People of Israel ***