What are the differences between Sunnis and Shias?
Muslims are split into two main branches, the Sunnis and Shias. The split originates in a dispute soon after the death of the Prophet Muhammad over who should lead the Muslims.
The great majority of Muslims are Sunnis - estimates suggest the figure is somewhere between 85% and 90%.
The two communities share fundamental beliefs - the "oneness" of Allah, that Muhammad was the last prophet, prayer, fasting and the pilgrimage to Mecca for example. But there are differences in doctrine, ritual, law, theology and religious organisation.
They also often seem to be in competition.
The Iranian revolution of 1979 launched a radical Shia Islamist agenda that laid down a theological and ideological challenge to conservative Sunni regimes, particularly in the Gulf.
In countries with large Shia communities, Shias often make up the poorest sections of society and see themselves as oppressed and discriminated against.
In many countries the two communities live separate lives. However, in Iraq intermarriage between Sunnis and Shia was common until recently.
In Lebanon, Shias have gained widespread respect and a strong political voice due to the political and military activities of Hezbollah.
Some conservative Sunni doctrines preach hatred of Shias.
Pakistan has a history of Shia-Sunni bloodshed dating back to the 1980s.
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 Shias, 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 Shias?
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 in a power struggle over who should be caliph, leader of the Muslims. His sons also struggled to capture the caliphate - Hussein died on the battlefield opposing a subsequent caliph and Hassan is believed to have been poisoned.
These events gave rise to the Shia cult of martyrdom and the rituals of grieving.
There is a distinctive messianic element to the faith and Shias 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 and, according to some estimates, Yemen. There are large Shia communities in Afghanistan, Azerbaijan, India, Kuwait, Lebanon, Pakistan, Qatar, Syria, Turkey, Saudi Arabia, and the UAE.