Short answer
Houris (hur al-'in) are the pure companions of Paradise described in the Quran as fair, with large beautiful eyes, like well-protected pearls. They are a special creation of Jannah for its people. Believing women are not left out: they will be in Jannah with their believing husbands, and no one there will be alone.
Belief in Jannah and what the Quran and authentic hadith describe in it, the houris included, is part of Sunni creed. Their exact nature is part of the unseen, so Muslims affirm what is reported without adding details.
The four schools agree on this.
Explanation
"Hur" refers to the striking contrast of the white and the dark of the eye, and "'in" to eyes that are wide and beautiful. The Quran describes them as "fair women with large, beautiful eyes, the likenesses of pearls well-protected" (56:22-23) and "reserved in pavilions" (55:72). Allah says He will marry the people of Jannah to them (44:54). A hadith in al-Bukhari adds that they are so radiant that the marrow of their shins is seen through the flesh.
The Quran also promises "purified spouses" (2:25) to all the people of Jannah, which scholars explain as spouses free from every impurity, illness and bad character. The famous number of seventy-two comes from a hadith in al-Tirmidhi about the special reward of the martyr; it is not a number given to every person.
A common question is what women receive. The Quran speaks of the believing men and women entering Jannah together, and says the people of Jannah will have there whatever their souls desire (41:31). A believing woman will be with her believing husband if both enter Jannah, and scholars have discussed the case of a woman who married more than once, holding that Allah will make her fully content. A hadith in Sahih Muslim says there will be no one in Jannah without a spouse. There is no jealousy, sadness or loneliness there.
The Quran's descriptions speak to people in words they know, but the reality is greater: Allah has prepared what no eye has seen and no heart has imagined. Read more in Jannah in Islam.
The evidence
Quran, Surah Al-Waaqia 56:22-23
وَحُورٌ عِينٌۭ ﴿٢٢﴾ كَأَمْثَٰلِ ٱللُّؤْلُؤِ ٱلْمَكْنُونِ ﴿٢٣﴾
And [for them are] fair women with large, [beautiful] eyes, The likenesses of pearls well-protected,
Describes the houris of Jannah.
Quran, Surah Ad-Dukhaan 44:54
كَذَٰلِكَ وَزَوَّجْنَٰهُم بِحُورٍ عِينٍۢ ﴿٥٤﴾
Thus. And We will marry them to fair women with large, [beautiful] eyes.
The people of Jannah will be married to houris.
Quran, Surah Al-Baqara 2:25
وَبَشِّرِ ٱلَّذِينَ ءَامَنُوا۟ وَعَمِلُوا۟ ٱلصَّٰلِحَٰتِ أَنَّ لَهُمْ جَنَّٰتٍۢ تَجْرِى مِن تَحْتِهَا ٱلْأَنْهَٰرُ ۖ كُلَّمَا رُزِقُوا۟ مِنْهَا مِن ثَمَرَةٍۢ رِّزْقًۭا ۙ قَالُوا۟ هَٰذَا ٱلَّذِى رُزِقْنَا مِن قَبْلُ ۖ وَأُتُوا۟ بِهِۦ مُتَشَٰبِهًۭا ۖ وَلَهُمْ فِيهَآ أَزْوَٰجٌۭ مُّطَهَّرَةٌۭ ۖ وَهُمْ فِيهَا خَٰلِدُونَ ﴿٢٥﴾
And give good tidings to those who believe and do righteous deeds that they will have gardens [in Paradise] beneath which rivers flow. Whenever they are provided with a provision of fruit therefrom, they will say, "This is what we were provided with before." And it is given to them in likeness. And they will have therein purified spouses, and they will abide therein eternally.
All the people of Jannah will have pure spouses.
Quran, Surah Fussilat 41:31
نَحْنُ أَوْلِيَآؤُكُمْ فِى ٱلْحَيَوٰةِ ٱلدُّنْيَا وَفِى ٱلْءَاخِرَةِ ۖ وَلَكُمْ فِيهَا مَا تَشْتَهِىٓ أَنفُسُكُمْ وَلَكُمْ فِيهَا مَا تَدَّعُونَ ﴿٣١﴾
We [angels] were your allies in worldly life and [are so] in the Hereafter. And you will have therein whatever your souls desire, and you will have therein whatever you request [or wish]
Every believer, man or woman, will have what they desire.
Sahih al-Bukhari 3245
Sahih... لِكُلِّ وَاحِدٍ مِنْهُمْ زَوْجَتَانِ يُرَى مُخُّ سُوقِهِمَا ...
"Each of them will have two wives, the marrow of whose shins is seen through the flesh because of their beauty."
Describes the purity and beauty of the companions of Jannah.
Sahih Muslim 2834
Sahih... وَمَا فِي الْجَنَّةِ أَعْزَبُ ...
"And there is no one in Paradise without a spouse."
No one in Jannah will be alone.
Jami' at-Tirmidhi 1663
Graded sahih... وَيُزَوَّجُ اثْنَتَيْنِ وَسَبْعِينَ زَوْجَةً مِنَ الْحُورِ الْعِينِ ...
"And he is married to seventy-two of the hur al-'in."
The source of the number seventy-two, which is about the martyr.
Where the ruling is recorded
- Hadi al-Arwah (Ibn al-Qayyim), Chapters on the hur al-'in and the women of Paradise. Gathers the Quranic verses and hadith on the companions of Jannah.
- al-Aqida al-Tahawiyya, Section on Paradise and Hell. Affirms that Jannah exists and its bliss never ends.
This answer is pending review by the scholars on our team. 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.