Both are branches of Islam and the adherents of both are Muslims, all bound by the same Quran, the same five pillars of Islam – belief in one God, daily prayer, fasting, charity, and hajj, or pilgrimage. Where they mainly differ is on the question of who should have succeeded the Prophet Muhammad, who founded Islam in 620.
Sunnis and Shiites differ on who should have succeeded Muhammad after his death in 632. (We're talking almost 1500 years ago people!) Sunnis supported the succession of Abu Bakr, the prophet’s friend; Shiite Muslims felt the rightful successor was the prophet’s son-in-law and cousin, Ali bin Abu Talib.
Sunnis believe Muslim leaders can be elected, or picked, from those qualified for the job. Shiites believe leaders should be direct descendants of the Prophet Muhammad. So they don’t recognize the same authority in Islam – kind of like the way Catholics and Protestants are all Christians and have the same Bible, but only Catholics recognize the authority of the pope. And like Catholics and Protestants, both Sunnis and Shiites have their own religious holidays, customs and shrines.
In the continuing conflict in Iraq, Kurds frequently are mentioned alongside Iraq’s Sunni and Shia Muslim populations as one of the key groups involved in power struggles for which sharp religious divides have played a major part. But while the Kurds are a crucial part of Iraq’s political makeup, they are an ethnic group, not a distinct religious sect within Islam. Kurds are more appropriately compared to Arabs, the largest ethnic group in Iraq, or other regional ethnic groups such as Assyrians or Turkmen.
One of the groups caught in the path of Sunni militants fighting under the banner of the Islamic State of Iraq and Syria (ISIS) are the Yazidis, Violence against Yazidis predates the current offensive, although ISIS represents a more systematic threat to the group.
The Yazidis are an ethnically Kurdish religious group whose beliefs include elements similar to Zoroastrianism, Christianity and Islam. They are mainly concentrated in northern Iraq, but some also live in Syria, Turkey and a few other countries.
If you're still confused, perhaps this 4 minute video may help clear everything up! It definitely helped me!!!