Sunni are the majority sect of the Mohammedan religion. They account for some 60% of the world's fastest-growing religion. Their major rivals in the world of Islam, are the Shia. Shi'ites are just under 40% of Islamic devotees. Saudi Arabia (with the major Islamic pilgrimage destinations of Mecca and Medina) is the epicentre of Sunni belief. Iran is the spiritual home of the world's Shia.
Like Christianity, the differences between sects are minor when viewed from the outside. Unlike Roman Catholics, Protestants do not believe in the Holy Trinity. The basic forms of worship in Roman Catholic and Protestant churches, are virtually identical - as is the worship performed by the world's Muslims.
Sunni and Shia disagree on the form of succession to the Prophet Mohammed. One sect claims that Mohammed's son-in-law Ali carried on the faith. The other branch of the faith maintains that a Caliphate, or committee of 3 holy men, received and handed down the holy writ from the Prophet before his death.
Who knows? All that is certain, is that a lot of deaths might have been avoided if people had just agreed to disagree. But then they'd have seen their power base disappear, wouldn't they? Ah, well - plus ca change, plus c'est la meme chose.