Muslims are split into two main branches, the Sunnis and Shia. The split originates in a dispute soon after the death of the Prophet Muhammad over who should lead the Muslim community.
The great majority of Muslims are Sunnis - estimates suggest the figure is somewhere between 85% and 90%.
Members of the two sects have co-existed for centuries and share many fundamental beliefs and practices.
Though they may not interact much outside the public sphere, there are always exceptions. In urban Iraq, for instance, intermarriage between Sunnis and Shia was, until recently, quite common.
The differences lie in the fields of doctrine, ritual, law, theology and religious organisation.
Their leaders also often seem to be in competition.
From Lebanon and Syria to Iraq and Pakistan, many recent conflicts have emphasised the sectarian divide, tearing communities apart.
Who are the Sunnis?
Sunni Muslims regard themselves as the orthodox and traditionalist branch of Islam.
The word Sunni comes from "Ahl al-Sunna", the people of the tradition. The tradition in this case refers to practices based on precedent or reports of the actions of the Prophet Muhammad and those close to him.
Sunnis venerate all the prophets mentioned in the Koran, but particularly Muhammad as the final prophet. All subsequent Muslim leaders are seen as temporal figures.
In contrast to Shia, Sunni religious teachers and leaders have historically come under state control.
The Sunni tradition also emphasises a codified system of Islamic law and adherence to four schools of law.
Who are the Shia?
In early Islamic history the Shia were a political faction - literally "Shiat Ali" or the party of Ali.
The Shia claimed the right of Ali, the son-in-law of the Prophet Muhammad, and his descendants to lead the Islamic community.
Ali was killed as a result of intrigues, violence and civil wars which marred his caliphate. His sons, Hassan and Hussein, were denied what they thought was their legitimate right of accession to caliphate. Hassan is believed to have been poisoned by Muawiyah, the first caliph (leader of Muslims) of the Umayyad dynasty.
His brother, Hussein, was killed on the battlefield along with members of his family, after being invited by supporters to Kufa (the seat of caliphate of Ali) where they promised to swear allegiance to him.in Istanbul
These events gave rise to the Shia concept of martyrdom and the rituals of grieving.
There is a distinctive messianic element to the faith and Shia have a hierarchy of clerics who practise independent and ongoing interpretation of Islamic texts.
Estimates of the number of Shia range from 120 to 170 million, roughly one-tenth of all Muslims.
Shia Muslims are in the majority in Iran, Iraq, Bahrain, Azerbaijan and, according to some estimates, Yemen. There are large Shia communities in Afghanistan, India, Kuwait, Lebanon, Pakistan, Qatar, Syria, Turkey, Saudi Arabia and the UAE.
What role has sectarianism played in recent crises?
In countries that have been governed by Sunnis, Shia tend to make up the poorest sections of society. They often see themselves as victims of discrimination and oppression. Some extremist Sunni doctrines have preached hatred of Shia.
The Iranian revolution of 1979 launched a radical Shia Islamist agenda that was perceived as a challenge to conservative Sunni regimes, particularly in the Gulf.
Tehran's policy of supporting Shia militias and parties beyond its borders was matched by the Gulf states, which strengthened their links to Sunni governments and movements abroad.
Image caption Discontent among the Shia has fuelled street protests in Bahrain
During the civil war in Lebanon, Shia gained a strong political voice because of the military activities of Hezbollah.
In Pakistan and Afghanistan, hardline Sunni militant groups - such as the Taliban - have often attacked Shia places of worship.
The current conflicts in Iraq and Syria have also acquired strong sectarian overtones. Young Sunni men in both countries have joined rebel groups, many of which echo the hardline ideology of al-Qaeda.
Meanwhile, many of their counterparts from the Shia community have been fighting for - or alongside - government forces.