Fard-i Ayn & Fard-i Kifaya

With respect to comprehensiveness, there are two kinds of fard [1]. When something is compulsory for each individual Muslim, it is called fard-i-'ayn. If the obligation of a fard lapses from other Muslims when one Muslim performs it, that is, if other Muslims do not have to perform it when it is carried out by one Muslim, and if all Muslims are responsible and become sinful when no Muslim performs it, this kind of fard is called fard-i-kifaya.

It is fard-i-'ayn for every man and every woman to learn the knowledge concerning the heart, that is, al 'ilm-i-akhlaq. For example, it is fard-i-'ayn for every Believer to learn that it is haram [2] to foster malices such as hiqd, which means 'to bear a grudge', hasad, [which means the feeling you have when you wish that the qualities or possessions that someone else has belonged to you instead of others. If you wish that you could have the same qualities and possessions and do not mind others' having them, too, it is not hasad; it is ghibta (envy, yearning), which may cause thawab [3]], kibr, [which means arrogance, conceit. It is not arrogance to assume an expression of arrogance towards an arrogant person. It causes the same thawab as if you gave alms], su'i zan, [i.e. having a bad opinion of someone who is good]. As it is seen, it is fard-i-'ayn to learn iman [4], i.e. the tenets of Sunni [5] belief in summary and to learn the good and bad habits.

It is fard-i-kifaya to learn how to perform the salat of janaza, the services to be done to a dead person, the technicalities and methods of arts and trade, (and science well enough to make and use modern weapons). In other words, these learnings are fard only for those who are involved in these activities; they are not fard for other people. However, if an adequate number of people do not learn them, all the other Muslims living in the same place, as well as the government, will become sinful.

By the same token, it is fard for a person who intends to be a doctor to receive high school and medical education, but it is not fard for a would-be engineer to study medicine. Also it is fard-i-'ayn for every Muslim to memorize one ayat [6]. It is wajib [7] to learn the (sura [8] called) Fatiha plus three ayats, or a short sura (instead of three ayats). It is fard-i-kifaya to memorize the entire Qur'an al-karim. Learning fiqh [9] more than you need is preferable to becoming a hafiz (committing the entire Qur'an to your memory.

[1] fard: an act or thing that is commanded by Allahu ta'ala in the Qur'an al-karim. Fard (or fard) means (any behaviour or thought or belief which is) obligatory. Islam's open commandments are called fard (pl. faraid).
[2] haram: an action, word or thought prohibited by Allahu ta'ala.
[3] thawab: Muslims will be rewarded in the Hereafter for all their pious actions which they have done in the world. The rewards which Muslims will be given in the Hereafter are called thawab. The word is used as an adjective as well as a noun. For example, when we say that an action is very thawab we mean that Allahu ta'ala will give many rewards for the action. Rewards which Muslims will be given in the Hereafter.
[4] iman: faith, belief, beliefs of Islam; kalam, i'tiqad.
[5] Sunni: (one) belonging to Ahl as-Sunna.
[6] ayat: A verse of al-Qur'an al-karim; al-ayat al-karima.
[7] wajib: (act, thing) never omitted by the Prophet, so almost as compulsory as fard and not to be omitted. Wajib al-wujud: Being whose existence is indispensable and nonexistence is impossible.
[8] suura(t): a Qur'anic chapter [a chapter of the Qur'an].
[9] fiqh: knowledge dealing with what Muslims should do and should not do; actions, a'mal, 'ibadat.