What is the Sunni of Muhammad?

Shortly after the death of Muhammad, there was a major split in the Islamic religion.  They split over who was the successor to the leadership of Islamic religion.

The Sunni Muslims rely on the writings of Muhammad in the Qur'an and other texts as the sole source of their religious doctrine.  They are the ones who have Sharia law, require their women to wear hijabs, etc.  They are more strict in their doctrines and continue to live like it is the 7th century.

While Shia Muslims look to their teachers (ayatollahs) to define their religion.  They supplement the Qur'an with the traditions and teachings from beyond the time of Muahmmad.  As such, they are less strict in their rules, and more open to redefine their religion in terms of current culture and society.  (Such as not having to wear 7th century clothing.)

The biggest denomination of Islam, They make most of the terrorist attacks and terrorist groups such as Al Qaeda, Boko Haram and ISIS, they all are Sunnis.