Short answer
Gog and Magog (Yajuj and Majuj) are two vast nations of human beings, held back behind a barrier built by Dhul-Qarnayn. Near the end of time the barrier will give way, they will pour out and spread corruption, and Allah will destroy them after Isa (Jesus) returns. Their release is one of the major signs of the Hour.
Belief in the emergence of Yajuj and Majuj is part of Sunni creed, because the Quran and sahih hadith state it. Al-Fiqh al-Akbar lists them with the Dajjal, the descent of Isa and the sun rising from the west among the signs of the Last Day that are true and will happen.
The four schools agree on this.
Explanation
The Quran mentions them twice. In Surah al-Kahf a people complained to Dhul-Qarnayn, a righteous ruler whom Allah gave power on earth, that Gog and Magog were spreading corruption in the land. He built a barrier of iron blocks with molten copper poured over it, which they could neither climb nor pierce, and he said it would stand until the promise of his Lord came (18:94-98). In Surah al-Anbiya their release is tied to the coming of the Hour: they will descend from every height (21:96).
The hadith add detail. The Prophet ﷺ once woke alarmed and said that an opening the size of a ring made by the thumb and forefinger had been made that day in their barrier. In the long hadith of al-Nawwas ibn Sam'an in Sahih Muslim, after Isa kills the Dajjal, Allah tells him that He has brought out servants of His whom no one can fight, so he should take the believers up to Mount Tur. Yajuj and Majuj then sweep across the land. Isa and the believers turn to Allah in dua, and Allah destroys them all.
Who are they? The scholars agree they are human beings from the children of Adam, and many said they descend from Nuh's son Yafith (Japheth). The hadith describe them as extremely numerous. Neither the Quran nor any sahih hadith gives the location of the barrier, so attempts to fix it on a map, or to say that Gog and Magog are a particular modern nation, are guesses. Many stories about their strange shapes come from Israelite reports and are not reliable.
The safe position is to believe what the texts state and leave the details to Allah. For the other major signs and their order, read Signs of the Day of Judgement, and see how to be protected from the Dajjal.
The evidence
Quran, Surah Al-Kahf 18:94
قَالُوا۟ يَٰذَا ٱلْقَرْنَيْنِ إِنَّ يَأْجُوجَ وَمَأْجُوجَ مُفْسِدُونَ فِى ٱلْأَرْضِ فَهَلْ نَجْعَلُ لَكَ خَرْجًا عَلَىٰٓ أَن تَجْعَلَ بَيْنَنَا وَبَيْنَهُمْ سَدًّۭا ﴿٩٤﴾
They said, "O Dhul-Qarnayn, indeed Gog and Magog are [great] corrupters in the land. So may we assign for you an expenditure that you might make between us and them a barrier?"
Names Gog and Magog and the barrier built against them.
Quran, Surah Al-Anbiyaa 21:96
حَتَّىٰٓ إِذَا فُتِحَتْ يَأْجُوجُ وَمَأْجُوجُ وَهُم مِّن كُلِّ حَدَبٍۢ يَنسِلُونَ ﴿٩٦﴾
Until when [the dam of] Gog and Magog has been opened and they, from every elevation, descend
Links their release to the approach of the Hour.
Sahih al-Bukhari 3346
Sahih... وَيْلٌ لِلْعَرَبِ مِنْ شَرٍّ قَدِ اقْتَرَبَ فُتِحَ الْيَوْمَ مِنْ رَدْمِ يَأْجُوجَ وَمَأْجُوجَ مِثْلُ هَذِهِ ...
"Woe to the Arabs from an evil that has come near. Today an opening like this has been made in the barrier of Yajuj and Majuj."
Shows the barrier is real and that it is being opened little by little.
Sahih Muslim 2937
Sahih... إِنِّي قَدْ أَخْرَجْتُ عِبَادًا لِي لاَ يَدَانِ لأَحَدٍ بِقِتَالِهِمْ فَحَرِّزْ عِبَادِي إِلَى الطُّورِ ...
"I have brought out servants of Mine whom no one has the power to fight, so take My servants to safety on the Mount."
Places their emergence after the return of Isa and the death of the Dajjal.
Where the ruling is recorded
- al-Fiqh al-Akbar (Imam Abu Hanifa), Section on the signs of the Day of Resurrection. Lists the Dajjal, Yajuj and Majuj, the sun rising from the west and the descent of Isa as true signs that will happen.
- Tafsir Ibn Kathir, Commentary on Surah al-Kahf, the story of Dhul-Qarnayn. Explains the barrier and warns that many reports about their appearance come from Israelite sources.
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.