The split between the Muslims was formed over the years when attempting to select caliphs. The third caliph was murdered after twelve years in power. Mohammed’s son-in-law, Ali, became the next caliph, although with much opposition, including that of one of Mohammed’s wives, and he was also murdered five years later. Next, Muawiya came to power in 661, beginning the dynasty known as the Umayyad Caliphate. By now the Shiites had fostered the belief that caliphs should only be from the line of Mohammed, while it simply did not matter to the Sunnis. Around 680 AD, the division officially occurred. Although the two parties agree on the basic tenets, they each eventually developed their own way of thinking and living.
One of the things they agree on is that Allah is one. God is the only god. Both also believe that Mohammed was the last prophet. They agree that one day Allah will resurrect all humans and question (not judge) their beliefs and actions. They believe that all of the “famous” sins such as murder, adultery, stealing, etc. are sins. Finally, they both agree on the five pillars of Islam. Otherwise they wouldn’t be Muslims, would they? But there are quite a few more disagreements with the two branches than agreements.
According to the Sunnis, Allah has a body, although it is not exactly human. They interpret parts of the Quran literally where it talks about his leg, face, and hand. The Shiites say Allah does not have a body. Shiites say that Allah will never be seen, and the Sunnis believe Allah can be seen, on earth and in the afterlife. Another theological difference is the Shiites believe Allah commands something because it is a good thing (and does not command something because it is bad). Sunnis think that because Allah orders it, it makes it good. So, according to the Sunnis, if Allah orders you to murder someone it is not a sin. Shiites believe that Allah does not do anything that does not have a purpose, while their counterparts say Allah does some things aimlessly. Another important item is that the Shiites say Allah knows what we will do but does not make us do it. Sunnis say he creates all our acts. Shiites also believe that all prophets are sinless. Sunnis are split: Are they sinless their whole life or just since the beginning of their ministry? Do all sins count, or only infidelity? Does he have to sin intentionally or can it be unintentionally? These are major theological differences, but there are also differences in culture.
One of the things Shiites do differently from Sunnis is that during prayer they put their head on a piece of hard clay instead of the mat. Also, they combine prayers so sometimes they only pray three times a day as opposed to five. Shiites are also permitted “fixed-term temporary marriages”, which is banned by the Sunnis even though it was allowed during Mohammed’s age.