As the map above illustrates (a somewhat different map that seeks to portray the same data is found here and at odds, for example, on the prevailing Islamic religious tendencies in Sudan), Islam is not monolithic, just as Christianity is not.
The way that Sunni Islam is practiced in Libya is not the same as the way it is practiced in Saudi Arabia, Somalia or Afghanistan, each of which have distinctive elements. The groupings shown above are sometimes called "madhhab" (schools of "jurisprudence," called untranslated "Fiqh") by Muslims. Shi'ite Islam can likewise be disaggregated into distinct branches, and there are other divisions as well.
A Brief Taxonomy Of Islamic Religious Group Divisions
The madhhab shown above in Sunni Islam are the Hanafi, Hanbali, Maliki, and Shafi'i. The branches of Shi'ite Islam shown are the Ismaili, Ja'fari, and the Zaidi. The Ibadi movement in Islam is not strictly Sunni or Shi'ite.
As a first approximation, it may be easiest to describe the madhhab with geolocators.
The Hanafi Sunni Islamic madhhab is the predominant form of Sunni Islam in South Asia, Central Asia, the Middle East (north of Saudi Arabia) and Egypt. It is the largest school of Sunni Islamic jurisprudence.
One of the more important movements, mostly within the Hanafi school, is the Barelvi movement of South Asia:
The Barelvi movement was started in 1880 to defend contemporary traditionalist Islamic beliefs and practices from the criticisms of reformist movements like the Ahl al-ḥadīth (people of hadith). This included a defence of many traditional practices and rites associated with popular Sufism. . . . To its followers the movement is known as Ahle Sunnat wal Jama'at ("People of the traditions of Muhammad and the broad community"), as a means to lay exclusive claim to be the legitimate form of Sunni Islam, in opposition to its reformist rivals like the Deobandi, Ahl al-ḥadīth and Darul-uloom Nadwatul Ulama movements. . . . India Today estimates that the vast majority of Muslims in India adhere to the Barelvi movement, and The Times (UK) writes that a majority of (South Asian) Muslims in the United Kingdom adhere to the movement as well. Similarly, the conservative think tank The Heritage Foundation gives such estimates for the vast majority of Sunni Muslims in Pakistan. . . . The Barelvi movement has taken a stance against Taliban movements in South Asia, organising rallies and protests in India and Pakistan, condemning what they perceive as unjustified sectarian violence. The Sunni United Council (SUC), an amalgamation of eight Sunni organizations, launched the Save Pakistan Movement to stem the process of Talibanisation. Terming the Taliban a product of global anti-Islam conspiracies, the leaders of SUC charged it with playing into the hands of the United States to divide Muslims and bring a bad name to Islam.
The founder of the Barelvi movement "issued fatāwā of apostasy against the founders of the Deobandi, Wahhabism, Shi'i and "Qadiani" (Ahmadiyya) sects of Islam. . . . "Not only did [he] obtain confirmatory signatures from other scholars in the subcontinent, he managed to get agreement from a number of prominent ulama in Mecca. That occurred in the first years of the twentieth century—long before the Al-Saud and their Wahhabi allies got control of the Haramayn. The feat was, nevertheless, stunning. The antipathy of the Deobandis toward the Ahl-i Sunnah on the emotional level becomes more comprehensible when Ahmad Riza's fatwa receives a full explication."
The Barelvi movement's beliefs concerning Muhammad the Prophet's nature parallel those of gnostic Christian views about Christ that emphasize his spiritual being and divinity, rather than his humanity. The fatwa issued by its founder would be on a par with saying that the Pope isn't really a Christian, and might be compared to Martin Luther's posting of his Ninety-five Theses that kicked off the Reformation in Europe.
At the other extreme within the Hanafi school of Islamic jurisprudence is the Deobandi movement:
The movement began at Darul Uloom Deoband (a madrasah) in Deoband, India, where its foundation was laid on 30 May 1866. . . . The Deobandi movement gained significant traction in the early 1900s, mainly due to the activities of its graduates. They, in many instances, played a key role in establishing similar institutions in other parts of the Indian subcontinent. Deobandis are considered to be within the confines of Sunni Islam (Ahl as-Sunnah wa’l-Jamā‘ah). They follow the Ash'ari and Maturidi schools of aqidah (creed). In fiqh (Islamic jurisprudence) they primarily follow the Hanafi school of law while they accept the validity of the remaining three schools of Sunni Islam, namely the Shafi`i, Maliki and the Hanbali schools. In the spiritual science of Tasawwuf (Sufism) they follow the Chishti, Naqshbandi, Qadiri and Suhrawardi orders. Since the 1920s the Deobandi apolitical stance has taken shape in the transnational movement Tablighi Jamaat, but Islamist trends such as those of Pakistan's Jamiatul Ulama-i Islam have also emerged from the ranks of the Deobandis. Deobandi thought, which originated in a north Indian town, has thus reached many countries, such as Pakistan, South Africa and the United Kingdom. According to The Times, about 600 of Britain's nearly 1,400 mosques are run by Deobandi affiliated scholars, and 17 of the country's 26 Islamic seminaries follow Deobandi teachings, producing 80% of all domestically trained Muslim clerics. . . . Deoband has been criticized for a failure to reconcile Islam with modern values. One particular topic which has sparked criticism are fatwas issued by Deobandi clerics regarding the behavior of women in the political and economic spheres. For example, in August, 2005, Deobondi clerics from Darul, Uloom, a Deoband seminary, issued a fatwa stating that while Muslim women may contest political elections, they must observe purdah while doing so. In May 2010, clerics from the same seminary issued a fatwa stating men and women cannot work together in public offices unless the women are properly clothed.
The Tablighi Jamaat are, in more familiar terms, low key, dispositionally puritanical Evangelical Muslims, mostly mission oriented preachers, with a similarly loose level of organization, who go on revival tours originally "in response to Hindu revivalist movements, which were considered a threat to vulnerable and non-practicing Muslims" who favor "personal communication for proselytizing" and de-emphasize formal denominational affiliations. The discourage "social enmeshments in customary and ceremonial rituals" such as elaborate weddings. "In the 1990s and 2000s, sporadic violence resulted from disputes over control of Pakistani mosques between Barelwi and Deobandi."
The Deobandi has faced a scandal somewhat akin to the Catholic indulgences scandal of the Reformation alleging that religious fatwas were bought rather than decided in accordance with Islamic law. Their movement is also notorious as a source of inspiration for many who are involved in Islamic terrorism.
Hanabali is the Sunni Islamic madhhab predominant in Saudi Arabia, and its most distinctive pronouncement, from the view of an outsider, is that women must cover their heads, faces and hands in public. It is the most conservative of the four schools and the one that relies on Hadith the most. It is the smallest of the four major school of Sunni Islamic jurisprudence in terms of numbers of adherents.
The Maliki Sunni Islamic madhhab is associated roughly with areas where Berbers are or were found in North Africa and Southwest Europe; it is also found in the United Arab Emirates, Kuwait, and in some parts of Saudi Arabia. Many Maliki differences in result on Islamic law are subtle details of prayer practice; methodologically, the Maliki resort to a larger body of precedent and tradition than other schools of Islamic jurisprudence.
The Shafi'i Sunni Islamic madhhab is common in East Africa and in the vicinity of East of South Asia. It is a significant minority school of jurisprudence in South Asia, Kuwait and Saudi Arabia. It is most notable for its assertion that Islamic law should not be extended by analogy or legal reasoning beyond its authoritative pronouncements and accepts a fairly narrow set of pronouncements as authoritative; thus, practices that are widely accepted in the Muslim community and not expressly contrary to the Qur'an should not be prohibited by Islamic law.
The Ibadi are most commonly associated with Oman and the island of Zanzibar in Tanzania. While it is not strictly either Sunni or Shi'ite, generally speaking, their doctrines are closer to conservative Sunni Islam, with a somewhat different narrative of the afterlife and a more strongly religiously bolstered division between members and nonmembers. But, they see Islamic leadership succession as having gone awry around the same time as the Shi'ias do (for somewhat different reasons), and share with the Shi'ia similar metaphysical beliefs regarding the creation of the Qur'an.
The branches in Shi'ia Islam reflect groups on different sides of succession disputes (and Shi'ia Islam itself broke with Sunni Islam over a succession dispute), with ideological and practice differences arising incidental to, or as a result of, those splits.
The Ja'fari branch of Shi'ite Islam (sometimes called the "Twelvers" aka Ithnā‘ashariyyah) is predominant in Iran and is the most common form of Islam for Shi'ite minorities, making up about 85% of all Shi'ite Muslims. For them, the last Iman, born in 868 CE, is a messianic figure who has not died and will return with Christ to restore all that is good to the world.
The Alevi of Turkey, are a particularly notable subset of Twelvers who make up more than a third of the population of Turkey and help to explain why Turkish Muslims are so much more liberal than, for example, Egyptian Muslims, on a wide variety of issues.
Alevis are sometimes categorized as part of Twelver Shi'a Islam, and sometimes as its own religious tradition, as it has markedly different philosophy, customs, and rituals. They have many Sufi characteristics and express belief in the Qur'an and the Shi'a Imams, but reject polygamy and accept religious traditions predating Islam, like Turkish shamanism. They are significant in East-Central Turkey. They are sometimes considered a Sufi sect, and have an untraditional form of religious leadership that is not scholarship oriented like other Sunni and Shia groups. They number around 25 million worldwide, of which 22 million are in Turkey, with the rest in the Balkans, Albania, Bosnia and Herzegovina, Azerbaijan, Iran and Syria.
The Alevi are arguably the most "liberal" of the Muslim groupings, adhering to principles including: "Humanism, love and respect for all people (“The important thing is not religion, but being a human being”), tolerance towards other religions and ethnic groups (“If you hurt another person, the ritual prayers you have done are counted as worthless”), respect for working people ("The greatest act of worship is to work”), and equality of men and women, who pray side by side."
The Zaidi branch of Shi'ite Islam (sometimes called the "Fivers") is associated with Yemen and bordering parts of Saudi Arabia, and don't recognize the legitimacy of the last seven of the Imans recognized by other Shi'ites.
The Ismaili, the second largest branch of Shi'ite Islam which takes the position that the hereditary chain of succession of Imans remains in tact from the time of the prophet to today, are most common in South Asia (and are also the branch of Islam to which the Druze have the closest historical links). The largest grouping of the Ismaili are the Nizari who see Aga Khan IV as the current successor, and the second most common are the Bohra, found in South Asia and Yemen, whose sects recognize different current non-hereditary spiritual leaders (whom they call the Da'i) due to past succession disagreements, with the predominant group of Bohra recognizing Dr Syedna Mohammed Burhanuddin as the current Da'i. The capacity of Ismaili Muslims to seek guidance from the unifying living spiritual leader of their sect is probably the most pervasive distinction between the Ismaili and other Muslims.
These broad "denominational" type divisions also fail to capture movements with Islam such as the poetic and spiritually oriented Sufi movement, which might be compared loosely to cross-denominational movements like the charismatic movement in Christianity, that are at odds with the subdued and legalistic religious orientation of other Muslims, with the movement called "Wahhabism" and "Salafism" with extends beyond particular jurisprudential schools at the other extreme:
Sufism is a mystical-ascetic form of Islam. By focusing on the more spiritual aspects of religion, Sufis strive to obtain direct experience of God by making use of "intuitive and emotional faculties" that one must be trained to use. Sufis usually considered Sufism to be complementary to orthodox Islam, however it has often been accused of being an unjustified Bid‘ah or religious innovation by the Salafi. One starts with sharia (Islamic law), the exoteric or mundane practice of Islam and then is initiated into the mystical (esoteric path of a Tariqah (Sufi Order). Some Sufi followers consider themselves as Sunni or Shi'a, while others consider themselves as simply 'Sufi' or Sufi-influenced. . . .
Salafism is in general opposed to Sufism and Shi'a Islam, which they regard as heresies. They see their role as a movement to restore Islam from what they perceive to be innovations, superstitions, deviances, heresies and idolatries.
The methodology predominates mainly in countries such as Saudi Arabia, the other Arabian Peninsula states and elsewhere. It has significant numbers of adherents in India and Pakistan where the movement is known as Ahl al-Hadith (People of Hadith). It is also growing in popularity in countries such as those in the western world; in particular the United Kingdom, the United States, Canada and in many other countries.
There are multiple different Sufi orders. This form of Islam is very prominent in Pakistan, and is common in Africa, Turkey, historically Islamic Europe (e.g. Albania and Kosovo), Central Asia and South Asia.
Politically, the key point is that Islam is not a unified, monolithic world force that will unite against the rest of the world on any points apart from the core Islamic religious doctrines shared by all Muslims like acceptance of the Qur'an as an authoritative and inspired source of religious wisdom.
In Sunni Islam and for Twelvers in Shi'ia Islam, there is not even a living accessible figure comparable to the Pope in Roman Catholicism, or the Daili Lama in Tibetan Buddhism, who is an undisputed supreme spiritual leader unique connection to the divine.
Muslims, of course, have religious leaders in their communities, but in most schools of Islamic thought these leaders of more like Methodist peachers or Rabbis, religious trained community leaders, but nothing more than faithful human beings who have studied hard and made a commitment to devote their lives to religious life, not near divine beings. An important subset of these leaders style themselves as canon lawyers and academics, and securing an opinion from one that you are right on a point of Islam is a bit like finding an economist who agrees with you; it may mean that you have a good basis for your position, but it rarely actually implies that here is a consensus among all Muslims on that point. Other Muslim leaders, particularly in the Sufi tradition, are inspired poetic figures, the sort of person that one might describe as an Islamic Jack Kerouac, whose moving, different way of looking at and describing the world moves listeners to new understandings themselves.
For someone who is interested in making sense of the history that caused the human landscape of the world to appear as it does today, these groupings can show connections that enhance and shed light upon the groups suggested by language and population genetics. Religion is often more strongly conserved than language, and religious scripture and prayers often persist in languages that have otherwise died long ago. Population genetic patterns that match a religious grouping may be explained by the same history that helps explain the religion's geographic expanse.
Recognizing these distinctions also encourages caution when it comes to extrapolating something that seems to have an Islamic religious basis in one place to another where a different version of Islamic practice prevails.