Because of Islam's great growth geographically in the first two centuries of its inception, there needed to be a larger set of Islamic laws capable of handling the different needs of Muslims throughout the Empire.  The Qur'an and the Hadith were not detailed enough to provide all the answers.  Therefore, in the 8th century A.D., there arose a school of legal experts who interpreted and applied Islamic principles to different situations throughout the Empire.  However, different scholars disagreed with these experts in various areas.  This led to a variety of legal schools of thought within Islam. 
     These different schools became different sects within Islam.  The largest of the sects is the Sunni which comprises about 90% of all Muslims.  The next two largest are the Shi'i and Sufi.  After these, there are numerous splinter groups which are often named after the individual scholars who began them:  Hanifa, after Abu Hanifa; Maliki, after Malik ibn Anas; Shafi'i, after Muhammad ibn Idris al-Shafi'i; Zaydi, after Zayd ibn Ali; the Nusayri, Ismaili, Murji'ah, etc.

Sunni Muslims
     Sunni Muslims  These are followers of the Hanifa, Shafi, Hanibal and Malik schools. They constitute a 90% majority of the believers, and are considered to be main stream traditionalists. Because they are comfortable pursuing their faith within secular societies, they have been able to adapt to a variety of national cultures, while following their three sources of law: the Qur'an, Hadith and consensus of Muslims.
     The Sunni emphasize the power and sovereignty of Allah and his right to do whatever he wants with his creation.  Strict determinism is taught.  Its rulership is through the Caliphate, the office of Muslim ruler who is considered the successor to Muhammad.  This successor is not through hereditary lineage.

Shi'ite Muslims 
     The Shi'ites (also known as the Ja'firi school) split with the Sunni over the issue of the successor to Muhammad.  This split occured after the assassination of the fourth caliph in 661.  Shi'ites believe that the successor to Muhammad should have been Ali, his son in law, and that subsequent successors should have been through his lineage through his wife Fatima.
     Shi'ism is broken into three main sects: the Twelve-Imam; Persia, Iraq, Afghanistan, Lebanon, Pakistan, and Syria), the Zaydis (Yemen), and the Ismailis (India, Iran, Syria, and East Africa).  Each group, of course, has differences of doctrine.
     "Shi'ite theology includes a doctrine known as the five supports:  these are Divine Unity (tawhid), prophecy (nubuwwah), resurrection of the soul and body at the Judgment (ma'ad), the Imamate1 (imamah), and justice ('adl).  The first three are found in Sunni Islam, albeit with some differences of emphasis; the Imamate, however , is the essence of Shi'ism, and the last, justice, is an inheritance from the Mu'tazilites, or rationalists, whose system is in many ways perpetuated in Shi'ite theology..."1  The Imamate, fom the word "Imam", in the Shi'ite traditions is the political and religious leader of the Shi'ite sect.  This person possess great power and influence.  According to Shi'ite doctrine, the Imam must be a biological successor of Ali.  The Imam is also sinless and infallible on all matters of Islamic doctrine and will intercede for Muslims in the afterlife.  The Shi'i and the Sunni differ in some interpretations of the Qur'an and Hadith and even have a different canon of Hadith and the Sunni.
    

Sufi Muslims
     The Sufi are a mystical tradition where the followers seek inner mystical knowledge of God. This sect "officially" developed around the 10th century and has since fragmented into different orders:  Ahmadiyya, Qadariyya, Tijaniyya, etc.  Of course, the Sufi believe their roots can be traced back to the inception of Islam in the early 7th century. 
     The Sufi mystic must follow a path of deprivation and meditation.  There are various forms of abstinence and poverty.  Worldly things are renounced and a complete trust in God's will is taught.  The goal is to attain to a higher knowledge and experience of Allah.  The mystical focus meant that the Qur'an could be interpreted in different ways and so Sufism taught that the Qur'an had mystical meanings hidden within its pages.  Out of this mysticism a type of pantheism developed among some Sufi believers.  Pantheism is the teaching that God and the universe are one.  Of course, the orthodox Muslims, called the Sunni, reject this idea since they claim that Allah is the creator of the universe and distinct from it.
     In part, Sufism arose as a reaction to the growing Islamic materialism that had developed in the Empire at that time.  Islam had achieved great power and geographical scope and with it, the material gain was great.  

Conclusion
     As you can see, Islam is not the united religious system it claims to be.  There are divisions among its ranks and even those divisions have divisions.  But what is interesting is that the Qur'an tells the Muslims to have no such divisions.

"The same religion has He established for you as that which He enjoined on Noah - the [sic] which we have sent by inspiration to thee - and that which we enjoined on Abraham, Moses, and Jesus:  namely, that ye should remain steadfast in religion and make no divisions therein:  to those who worship other things than Allah, hard is the (way) to which thou callest them..." (42:13)

     If this is the case, then the Muslim must admit that the divisions within Islam are sinful.  But, such is the nature of humanity, to divide and set ourselves against one another.