NEW YORK • Sufism is a mystical form of Islam, a school of practice that emphasises the inward search for God, and shuns materialism.
It has produced some of the world's most beloved literature, like the love poems of the 13th-century Iranian jurist Rumi.
Its modern-day adherents cherish tolerance and pluralism, qualities that, in many religions, unsettle extremists.
While it is sometimes misunderstood as a sect of Islam, it is actually a broader style of worship that transcends sects, directing followers' attention inward.
Sufi practice focuses on the renunciation of worldly things, purification of the soul and the mystical contemplation of God's nature.
Followers try to get closer to God by seeking spiritual learning known as tariqa.
The Islamic State in Iraq and Syria (ISIS) targets Sufis because it believes that only a fundamentalist form of Sunni Islam is valid.
Some fundamentalists see the reverence for saints, which is common in Shi'ite Islam, as a form of idolatry as, in their view, it shows devotion to something other than the worship of a singular God.
Some consider Sufis to be apostate because saints were not part of the original practice of Islam at the time of the Prophet Muhammad, who died in 632.
Experts say amicable relations between Sufis and the Egyptian government may also be a factor, giving the attack a political dimension.