You have a very good question. The media seems to portray Islam as monolithic and many Americans have the same view. In reality there are a few different denominations.
Here are the explanations of a few as found on
http://www.religionfacts.com/islam/sects.htm Sunni (Sunnah) Islam
With 940 million adherents, Sunni Islam is the largest denomination of Islam. Followers of the Sunni tradition are known as Sunnis or Sunnites; they sometimes refer to themselves as Ahlus Sunnah wal-Jamaa'h, "adherents to the Sunnah and the assembly."
Sunnis have their historical roots in the majority group who followed Abu Bakr, an effective leader, as Muhammad's successor, instead of his cousin and son-in-law Ali. The Sunnis are so named because they believe themselves to follow the sunnah or "custom" of the Prophet.
Sunnis base their religion on the Quran and the Sunnah as understood by the majority of the community under the structure of the four schools of thought. The four Sunni schools of law (madhahib) - the Hanafi, the Maliki, the Shafi'i and the Hanbali - are sometimes mistakenly understood as different sects, but they are not. These four schools of religious law associate themselves with four great scholars of early Islam: Abu Haneefah, Malik, Shafi'i, and Ahmad bin Hanbal. These scholars were known for their knowledge and piety throughout the Muslim world. They differed only in minor issues of application of certain principles in the religion and were not in opposition to each other. In fact, Ahmad bin Hanbal was a student of Shafi'i, who was a student of Malik.
Sufism is considered by the majority of Sunnis to be integral part of Sunni Islam. It deals with the spiritual aspects of a Sunni Muslim's everyday life. Some of the most famous Sufi schools are the Qadiri, Naqshbandi, Shadhili, Chishti, and Rifa‘i paths or tariqas.
Sunnis view the Shi'ites as from the ahlul-bidah; i.e., the people of innovation. Sunnis oppose Shi'ite beliefs concerning some of the companions of the Prophet, the belief in the Imamate and difference on the Caliphate, and others. Other groups considered to be outside Islam by Sunnis are Nation of Islam, Ahmadiyya, and Ismailis.
Shi'a Islam
Shia Islam includes most of the Muslims that are not counted among the Sunni. The primary division within Islam, between Sunni and Shi'a, dates to the death of the Prophet Muhammad when his followers were faced with the decision of who would be his successor as the leader of Islam. Shi'ites are those who followed Ali, the closest relative of Muhammad, as Muhammad's successor. Today there are approximately 120 million Shi'ite Muslims in the world.
The Shia consist of one major school of thought known as the Jafaryia or the "Twelvers," and a few minor schools of thought, as the "Seveners" or the "Fivers." These names all refer to the number of imams they recognize after the death of Muhammad. The term Shi'a is usually meant to be synonymous with the Jafaryia/Twelvers.
Sufism
Sufism is less an Islamic sect and more an alternative way of approaching the Islamic faith. Sufis are mystically-inclined Muslims who seek a direct experience of God. Sufism has been a prominent movement within Islam throughout most of its history. By the 19th century CE, more than half of the male Muslim population was attached to a Sufi order (tariqa).
Ahmadiyya
Ahmadiyya Islam was founded in 1889 by Mirza Ghulam Ahmad (c. 1839-1908) in Qadian, Punjab, India. Ahmad claimed to be the appearance of the Promised Messiah (or, according to some sources, a reappearance of the Prophet Muhammad) as well as an incarnation of the Hindu god Krishna.
Ghulam Ahmad taught that Jesus feigned his crucifixion and resurrection, then lived to be 120 years old in India, contradicting the orthodox Muslim doctrine that Jesus was taken up into heaven before his death. Ahmad also reinterpreted jihad as a nonviolent battle against nonbelievers, using as its weapon the pen instead of the sword. These doctrines, along with the teaching that Ahmad was a prophet like Muhammad, have led Ahmadiyyas to be denounced as heretics by most of orthodox Islam.
Upon the death of Ahmad, Mawlawi Nur-ad-Din was elected as successor (caliph). When he died in 1914, the Ahmadiya group split into two groups.
Ahmadiyya Islam is also associated with several Sufi orders, most notably the Al-Badawi order of Egypt, named for an Islamic saint who died in 1276.
If you wish, I can supply some websites for more information on the differnt groups.
Peace