The division between Shia and Sunni dates back to the death of the Prophet Muhammad in 632. This event raised the question of who was to take over the leadership of the Muslim nation.

Sunnism is the largest and most orthodox branch of Islam. The word Sunn, in Arabic, comes from a word meaning "one who follows the traditions of the Prophet."

Sunni Muslims agree with many of the Prophet's companions at the time of his death: that the new leader should be elected from among those capable of the job. For example, following Prophet Muhammad's death, his close friend and adviser, Abu Bakr, became the first Caliph (successor or deputy of the Prophet) of the Islamic nation.

On the other hand, some Muslims believe that leadership should have stayed within the Prophet's family, among those specifically appointed by him, or among Imams appointed by God Himself.

Shia Muslims believe that following the Prophet Muhammad's death, leadership should have passed directly to his cousin and son-in-law, Ali bin Abu Talib. Throughout history, Shia Muslims have not recognized the authority of elected Muslim leaders, choosing instead to follow a line of Imams they believe have been appointed by the Prophet Muhammad or God Himself.

The word Shia in Arabic means a group or supportive party of people. The commonly-known term is shortened from the historical Shia't-Ali, or "the Party of Ali." This group is also known as Shiites or followers of Ahl al-Bayt or "People of the Household" (of the Prophet).

Within the Sunni and Shia branches, you can also find many sects. For example, in Saudi Arabia, Sunni Wahhabism is a prevalent and puritanical faction. Similarly, in Shiitism, the Druze are a somewhat eclectic sect residing in Lebanon, Syria, and Israel.

Source: https://www.learnreligions.com/difference-between-shia-and-sunni-muslims-2003755

I'm Sunni btw