There's an interesting article in the New York Times about the differences between the Shia and Sunnis in Iraq. Following is an excerpt of Ancient Rift Brings Fear on Streets of Baghdad:
Shiites split off from Sunnis after the Prophet Muhammad died in the seventh century. That created a crisis over who would succeed him as leader of the Muslim community. One group of Muslims chose Muhammad's friend, Abu Bakr. They would become the Sunnis, a vast majority of the world's Muslims.
A smaller group believed the rightful successor was Ali, the prophet's son-in-law and cousin. They would become the Shiites, who today are concentrated in India, Pakistan and Persian Gulf countries. Abu Bakr won out, though after he died Ali eventually became caliph. He was assassinated, and the Muslim community began to splinter. Ali's son Hussein led a rebellion but he, too, was cut down, in a battle in Karbala, Iraq. Hussein's death was the beginning of Shiism and it started a culture of martyrdom, evident each year during a festival in Karbala when Shiites whip and cut themselves to symbolize Hussein's pain.
Over the years, the rivalry between the partisans of Ali and those who supported Abu Bakr evolved into two schools of theology. For example, when it comes time to pray, Shiites believe a person's arms should be straight; most Sunnis say they should be bent. Shiites allow temporary marriage; Sunnis say it is forbidden. In some cases, Shiite inheritance law is more generous to women than is Sunni inheritance law.
Shiites follow ayatollahs, or supreme jurists, who some believe have divine powers. Sunni Islam is more decentralized among local imams.
Southern Iraq is essentially the center of Shiite Islam, with holy shrines in Karbala, Kufa and Najaf. The Sunni Arabs are concentrated in the west, especially in Anbar Province, the heartland of Iraqi tribal culture. In Baghdad and eastern cities like Baquba, the populations are mixed, while in the north, Sunni Kurds predominate.
In Iraq, tribal identity is also important, and many people use tribal names as last names. Because certain tribes are rooted in certain areas, a last name like Saidi, Maliki or Kinani may be typically Shiite, while names like Zobi, Tikriti and Hamdani are typically Sunni.
Certain first names may also reveal sect: Omar and Othman are Sunni names; Haidar and Karrar are Shiite ones.
Dress, too, can be a sign, but again not because it has religious significance. In western Iraq, the favored headdress is white and red; in the south it is white and black.
Note: The part on "last names" is a bit misleading. Muslims don't use "last names" or surnames as Westerners do. We use a type of patronymic, similar to that used by Hindus and Russians. The word "bin" means "son of" and "bint" or "binte" means "daughter of." So, with the American expat blogger Bin Gregory, his name is not "Bin" or "Greg." He's saying that he is the son of his father, Gregory. (I do know his Muslim name, but I'm not revealing it here.)
Some of the "last names" mentioned in the article indicate the city or area where the person is from. For example, "Tikriti" is mentioned above. Saddam Hussein's formal name is "Saddam bin Hussein Abd al-Majid al-Tikriti." He is Saddam, son of (his father) Hussein Abd Al-Majid, the Tikriti (or person from the Iraqi town of Tikrit). It is a similar practice to that mentioned in the Bible; e.g., Joseph of Arimathea.