Shia and Sunni are two branches of Islam, which is the main religion in many countries in the Middle East. Shia Muslims and Sunni Muslims have some differences in how they practice Islam.
Sunnis are the largest group of Muslims and they believe that the first four caliphs (successors of the Prophet Muhammad) were the rightful leaders of the Muslim community after the Prophet died. Sunnis also believe that any Muslim can become a caliph if they follow the example of the Prophet.
Shia Muslims believe that the rightful leader of the Muslim community should be a family member of the Prophet Muhammad. They specifically believe that the fifth caliph, Ali, was the true successor of the Prophet and that the previous four were not rightful leaders. They also have some different customs and beliefs in how they practice Islam.