The mystical tradition within Islam, which has profoundly enriched both Sunni and Shia Islam since the 8th century CE. It has played an extraordinary role in the expansion and spiritual development of Islam.
Traditionally, Sufis organise themselves in orders, following chains of spiritual and mystical authority.
A number of "non-traditional" Sufi schools/organizations have also developed, mainly (but not exclusively) in the West. They tend to see Sufism as a universal spiritual quest/philosophy predating Islam, independent of the Qur'an and the teachings of Prophet Muhammad; as a result, they are much more open to other spiritual traditions.