Shia and Sunni Muslims use much the same texts for their worship and Shia is the next largest Islamic sect after Sunni Islam. However, 85 percent of all worshippers are Sunni. The nexus of Shia Islam is in Iran – Persia – and differences between the two Islamic sects were initially political and harken back to the death of the Prophet Muhammad when Muhammad's personal friend Abu Bakr became the first Caliph of the Islamic nation. Shia Muslims believe leadership should have stayed in the family and the mantle should have been passed to Muhammad's son-in-law, Ali.
Thus it is that Shia Muslims don't obey Sunni Muslim leaders, but follow Imams appointed by Muhammad and God Himself. Shias are also known as followers of "Ahl-al-Bayt" or "People of the Household" (of the Prophet). Out of political differences have grown spiritual ones.
Shia Muslims elevate Imams as descendents from the household and believe them to be without sin and even divine. From the Sunni point of view, these Imams constitute a privileged caste; leadership is not divinely ordained but ought to be generated from what someone deserves.
Many other somewhat minor issues stem from what Shia and Sunnis recognize to begin with. Since Shia Muslims don't recognize the legitimacy of Muhammad's chosen successor, neither do they recognize various dictates generated by those successors. This has led to differences in terms of how the religion is observed – including differences in fasting, prayer and other religious habits.
But the main issue of the Shia/Sunni split is the emphasis on the Imams themselves and their potential divinity. The Sunni sect is by far the more democratic in the sense that Sunnis believe that religious leadership is primarily a function of the individual's spirituality and fitness to lead.