1. The word "Jinn"
The Arabic word jinn (جِنٌّ) comes from the word janna (جَنَّ) which means "to hide or conceal." Thus, the jinn are a species that are typically hidden from sight.
The singular word for jinn is jinni (جِنِّي). Another word synonymous in use to jinn is jann (جَانٌّ).
The word jinni is not mentioned in the Quran, but both jinn and jann are mentioned:
Say, "It has been revealed to me that a group of the jinn listened and said, 'Indeed, we have heard an amazing Quran (72:1).'"
Then on that Day none will be asked about his sin among men or jann (jinn) (55:39).
The word jinnah is also used to mean jinn:
From among the jinnah (jinn) and mankind (114:6).
It's worth noting that the jann can also refer to the father of the jinn (first of its race) and can also mean snake:
'Abdullah ibn 'Amr said, "The jinn, children of Jann, were on earth two thousand years before the creation of Adam. [Tafsir Al-Quran Al-'Azim by Ibn Kathir]
And [he was told], "Throw down your staff." But when he saw it writhing as if it was a jann (snake), he turned in flight and did not return. [Allah said], "O Moses, approach and fear not. Indeed, you are of the secure (28:31)."
2. Origin
The origin of the jinn is fire:
And the jinn We created earlier from scorching fire (15:27).
And He created the jinn from a flame of fire (55:15).
The angels were created from light, the jinn from a flame of fire and Adam from what was already described to you. [Muslim]
However, it is the view of the majority of scholars that the relation of jinn to fire is like the relation of man to earth (soil). Although Adam was created from the earth, his descendants are created through a different means. Similarly, the first jinn was created from fire but his descendants are created through a different means.
3. Time of creation
The jinn were created before mankind:
And We did certainly create man out of clay from an altered black mud. And the jinn We created before from scorching fire (15:26-27).
'Abdullah ibn 'Amr said, "The jinn, children of Jann, were on earth two thousand years before the creation of Adam. [Tafsir Al-Quran Al-'Azim by Ibn Kathir]
4. Visibility and forms
In general, the jinn in their natural forms are invisible to man but visible to some animals:
Indeed, he (iblis) sees you, he and his tribe, from a position where you cannot see them (7:27).
When you hear the barking of dogs and the braying of donkeys at night, take refuge with Allah, for they see which you do not see. [Abu Dawud]
However, the prophets are able to see them in their natural form:
Allah's Messenger (ﷺ) stood up (to pray) and we heard him say, "I take refuge in Allah from you." Then he said, "I curse you with Allah's curse" three times, then he stretched out his hand as though he was taking hold of something. When he finished the prayer, we asked, "O Messenger of Allah, we heard you say something during the prayer which we have not heard you say before, and we saw you stretch out your hand. He replied, "Allah's enemy iblis came with a flame of fire to put it in my face, so I said three times, "I take refuge with Allah from you." Then I said three times, "I curse you with Allah's full curse." But he did not retreat (on any one of these) three occasions. Thereafter I meant to seize him. I swear by Allah that had it not been for the supplication of my brother Sulaiman he would have been bound, and made an object of sport for the children of Al-Madinah. [Muslim]
It is worth noting the jinn in their natural forms are invisible to man not necessarily only because of their natural forms, but perhaps more so because they are from the ghaib (an unseen parallel realm). At the time of death, for example, the barrier to that realm is lifted and it is in clear view — the dying person sees that which the living around him do not see.
Some of the jinn can adopt human and animal forms. When in these states, the jinn can be seen by man:
Abu Hurairah said: Allah's Messenger (ﷺ) put me in charge of the Zakat of Ramadan (i.e. Zakat al-Fitr). Someone came and began scooping some of the foodstuff of (Zakat) with both hands. I caught him and told him that I would take him to Allah's Messenger (ﷺ). Then AbuHurairah mentioned the incident to the Prophet saying, "He (i.e. the thief) [bargained his release and] said, 'Whenever you go to your bed, recite the Verse of Al-Kursi (2:255) for then a guardian from Allah will be guarding you, and satan will not approach you till dawn.'" On that the Prophet (ﷺ) said, "He told you the truth, though he is a liar, and he (the thief) was satan." [Al-Bukhari]
There are jinn in Al-Madinah who have become Muslim. When you see one of them, call out to it [to leave] for three days. If it appears after that, then kill it, for it is a shaytan. [Muslim]
5. Nutrition and pets
The jinn eat, drink and even have pets:
They (the jinn) asked him (the Prophet) about their nourishment and he said, "You may have as meat every bone that falls in your hand on which the name of Allah was recited, and for animals every animal dropping." The Messenger of Allah (ﷺ) said, Don't wipe yourselves clean (istinja) with these things for these are the food of your brothers (jinn). [Muslim]
When any of you eats, he should eat with his right hand, and when he drinks, he should drink with his right hand, for the devil eats with his left hand and drinks with his left hand. [Muslim]
6. Gender and children
There are male and female jinns, and they capable of having sexual relations and bearing offspring:
And there were men from mankind who sought refuge in men from the jinn, so they [only] increased them in burden (72:6).
O Allah, I take refuge with You against the evil male and female jinn. [Al-Bukhari]
Umar asked the fortune teller, "What is the strangest thing your female jinn told you?" [Al-Bukhari]
And [mention] when We said to the angels, "Prostrate to Adam," and they prostrated, except for iblis. He was of the jinn and disobeyed the command of his Lord. Then will you take him and his descendants as allies other than Me while they are enemies to you? Wretched it is for the wrongdoers as an exchange (18:50).
[Maidens of Paradise] whom no man or jinn had deflowered (55:74).
7. Categories of jinn
The Prophet (ﷺ) categorised jinn into three broad categories:
There are three types of jinn: a type that possesses wings and flies in the air, another type which exist as [or in the form of] snakes and dogs, and a third type which resides in one place or wanders about. [Al-Tabari]
The powerful jinn are referred to as ‘ifrit (عِفْرِيتٌ). They possess enhanced abilities such as super strength and speed. For example, Allah said of one of them:
A powerful one ('ifrit) from among the jinn said, "I will bring it to you before you rise from your place, and indeed, I am for this [task] strong and trustworthy (27:39)."
The lowest and weakest jinn are referred to as hinn (حِنٌّ). According to Lisan Al-'Arab:
Hinn are the lowest and weakest of the jinn. Among them are those that take the form of pitch-black dogs.
The jinn that appear in terrifying, rapidly changing forms in the presence of humans are referred to as ghul (غُولٌ):
If the ghilan (sing. ghul) appear and transform themselves before you, call the adhan. [Ahmad]
The jinn may be categorised even further depending on their features and capabilities.
8. Muslim and non-Muslim jinn
Just like humans the jinn too have religions:
[The jinn said:] And among us are Muslims, and among us are the unjust. And whoever has become Muslim those have sought out the right course. But as for the unjust, they will be, for Hell, firewood (72:14-15).
9. Death
The jinn die as do humans, generation after generation:
Those are the ones upon whom the word has come into effect, [who will be] among nations which had passed on before them of jinn and men. Indeed, they [all] were losers (46:18).
I take refuge (with You) by Your Honour, none has the right to be worshipped but You, the One Who does not die while jinn and man die. [Al-Bukhari]