The difference between Shia and Sunni Muslims goes back to the decades after the death of the Prophet Muhammad. After the death of the prophet, the Muslim community had to choose a new leader. Many expected that Ali, Muhammad’s brother-in-law, was the most likely successor. But instead the Muslim community chose Abu Bakr to succeed Muhammad. Ali was the fourth successor to Muhammad, but was killed in battle. Those who believed Ali should be the rightful successor became the Shia, a term that originally just meant “party” or “faction.” Many Shia believed that others had usurped their authority in Islam.
Where the Sunni and Shia further split was that the Shia believed that Ali’s successor should be his son Husain. What ended up happening, however, was that a figure named Muawiyya took over the Muslim community. A conflict broke out between the two factions, and in a bloody battle – a massacre, really – Husain and his followers were slaughtered at Karbala, in modern Iraq. Today Shia communities commemorate this tragic event as a symbol of injustice and identify themselves with this early community of Shias. Most also believe that the descendants of Ali should have been the rightful heirs to Muhammad’s authority over the Muslim community. These leaders are called Imams and are believed to be divinely inspired. The Sunnis also use the term “imam,” but in that sense it just means a religious leader.
This is perhaps the biggest difference between Shia and Sunni, though there are some other differences in terms of law and philosophy. Also, most Muslims are Sunni, and Shia are a minority of about 15% of all Muslims, though they are majorities in Iraq and Iran. What this means is that until recent times, Shias were often a political minority, and suffered oppression, which also had the effect of strengthening the community and its sense of connection to the injustices Husain and the early Shia community suffered.