The Shiites (or Shi'ites or Shia) is one of two major branches of Islam. A schism in Islam occurred in A.D. 660, almost 30 years after the death of Muhammad. The Sunnis, the largest group, disagreed with the Shiites over who should take over the caliphate or leadership of the nascent Islamic community. Those supporting the Prophet's son-in-law, Ali, were called the Shi'at Ali or "Party of Ali" from which the name originates. The Shiite branch of Islam split further into three sub-branches: Zaidi, Ismai'ili(Sevener), and Imamiyya(Twelver). Shiites believe in a line of succession branching off from Muhammad's grandchildren, Hasan and Husein, and is relevant to the names of the Shiite branches. Shiites comprise about one tenth of all Muslims and are the majority faith in Iran and Iraq.
Following the Iranian Revolution in 1979 and the establishment of the theocracy of Ayatollah Khomeini, Iranian Twelver Shiite Islam became associated with Islamic fundamentalism and violent hostility to the West both politically and culturally. The Sunnis took inspiration from the Iranian Revolution and wanted to overthrow a Western-aligned government to create their own Islamic State under shariah law. The Afghan jihad was their first attempt.
It must be remembered that al Qaeda and the Taliban are entirely Sunni movements. Much of the teachings for groups like Al-Qaeda comes from the fiercely fundamentalist Wahhabi sect, and is as hostile to Shiite Islam as it is to Christianity and Judaism.
Wahhabi ideology is at war with itself and the whole world. The ruling Saudi klan not only claims Wahhabist traditions, its bloodline is intermarried with Ibn al-Wahhab, the founder of Wahhabi teachings. The Salafi-jihadi founders of the Islamic State, who also adhere to Wabbist traditions, claim the Saudi ruling family has become too corrupt and Westernized, and seek to remove them as the Custodians of the Islamic holy places of Mecca and Medina.
In other words, there is an establishment Wahhabi state religion in Saudi Arabia, and a radical Wahhabi movement outside Saudi Arabia with many exiles who seek to overthrow the House of Saudi and re-establish the Caliphate over Mecca. Disputes over Islamic leadership are as old as Islam itself, as represented by the Shia-Sunni split.