Who are Sufi Muslims and why do some extremists hate them?

Sufism is a mystical form of Islam, a school of practice that emphasizes 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.