Short answer
Jahannam is the Fire Allah has prepared for those who reject Him and for sinners He does not forgive. The Quran describes seven gates, a fuel of people and stones, and skins that are replaced so the punishment continues. The Prophet ﷺ said our fire is one part of seventy of its heat. It exists now, and the disbelievers' punishment in it never ends.
Sunni creed holds that Jannah and Jahannam are already created and will never cease to exist. Those who die in disbelief remain in the Fire forever; believers who enter it for their sins are eventually brought out by Allah's mercy and by intercession.
The four schools agree on this.
Explanation
The Quran describes it often, as a warning that is itself a mercy. "Indeed, Hell is the promised place for them all. It has seven gates; for every gate is of them a portion designated" (15:43-44). Its fuel is "people and stones", and over it are angels, harsh and severe (66:6). "Every time their skins are roasted through We will replace them with other skins so they may taste the punishment" (4:56). Its food includes the tree of zaqqum (44:43-44).
The Prophet ﷺ said: "Your fire is one part of seventy parts of the fire of Jahannam." He also said the one with the lightest punishment on the Day of Resurrection will have two embers under the soles of his feet from which his brain boils, and he will think no one is punished more than him.
The Quran uses several names for it: Jahannam, al-Nar (the Fire), al-Jahim, Saqar, al-Sa'ir, Laza, al-Hutamah and al-Hawiyah. It also says the hypocrites will be in its lowest depth (4:145), which shows it has levels. Lists online that rank these names as seven fixed floors come from later reports, not from a sahih text; what the Quran gives is seven gates.
Who stays forever? Those who die rejecting Allah after the message reached them. A believer who dies with sins may be forgiven outright, or punished for a time and then taken out; see will Muslims stay in Hell forever? Also read the Sirat bridge and Jannah.
The evidence
Quran, Surah Al-Hijr 15:43-44
وَإِنَّ جَهَنَّمَ لَمَوْعِدُهُمْ أَجْمَعِينَ ﴿٤٣﴾ لَهَا سَبْعَةُ أَبْوَٰبٍۢ لِّكُلِّ بَابٍۢ مِّنْهُمْ جُزْءٌۭ مَّقْسُومٌ ﴿٤٤﴾
And indeed, Hell is the promised place for them all. It has seven gates; for every gate is of them a portion designated."
Describes the seven gates of Hell.
Quran, Surah At-Tahrim 66:6
يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ قُوٓا۟ أَنفُسَكُمْ وَأَهْلِيكُمْ نَارًۭا وَقُودُهَا ٱلنَّاسُ وَٱلْحِجَارَةُ عَلَيْهَا مَلَٰٓئِكَةٌ غِلَاظٌۭ شِدَادٌۭ لَّا يَعْصُونَ ٱللَّهَ مَآ أَمَرَهُمْ وَيَفْعَلُونَ مَا يُؤْمَرُونَ ﴿٦﴾
O you who have believed, protect yourselves and your families from a Fire whose fuel is people and stones, over which are [appointed] angels, harsh and severe; they do not disobey Allah in what He commands them but do what they are commanded.
Describes the fuel and the angels over it.
Quran, Surah An-Nisaa 4:56
إِنَّ ٱلَّذِينَ كَفَرُوا۟ بِـَٔايَٰتِنَا سَوْفَ نُصْلِيهِمْ نَارًۭا كُلَّمَا نَضِجَتْ جُلُودُهُم بَدَّلْنَٰهُمْ جُلُودًا غَيْرَهَا لِيَذُوقُوا۟ ٱلْعَذَابَ ۗ إِنَّ ٱللَّهَ كَانَ عَزِيزًا حَكِيمًۭا ﴿٥٦﴾
Indeed, those who disbelieve in Our verses - We will drive them into a Fire. Every time their skins are roasted through We will replace them with other skins so they may taste the punishment. Indeed, Allah is ever Exalted in Might and Wise.
Shows the punishment does not ease.
Sahih al-Bukhari 3265
Sahih... نَارُكُمْ جُزْءٌ مِنْ سَبْعِينَ جُزْءًا مِنْ نَارِ جَهَنَّمَ ...
"Your fire is one part of seventy parts of the fire of Jahannam."
Shows how much fiercer it is than any fire on earth.
Sahih al-Bukhari 6562
Sahih... إِنَّ أَهْوَنَ أَهْلِ النَّارِ عَذَابًا يَوْمَ الْقِيَامَةِ رَجُلٌ عَلَى أَخْمَصِ قَدَمَيْهِ جَمْرَتَانِ ...
"The one with the lightest punishment among the people of the Fire on the Day of Resurrection is a man with two embers under the soles of his feet."
Shows that even the lightest punishment is severe.
Where the ruling is recorded
- al-Fiqh al-Akbar (Imam Abu Hanifa), Section on Paradise and Hell. States that Paradise and Hell are created, exist now and will never come to an end.
- al-Aqida al-Tahawiyya, Section on Paradise and Hell. Affirms that both are created and never perish.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.