A brand of either Islam and Buddhism. Sufi teaching tends to be very, very esoteric, making it difficult to assess from the outside.
Sufi teaching differs from typical religious teachings in that it was developed as a spiritual system that would teach only what corresponds with reality, and requires little of what some might call blind faith. Absent are lofty concepts of Heaven or Hell, in this way Sufi teachings are very Gnostic.
Much of the basic Sufi teaching is reflected in the (relatively) modern work of Mr. Gurdjieff.
Both Islamic Sufism and Buddhist Sufism are notoriously tolerant, and quite concerned with acquiring ever more knowledge. Unlike some organized religion, Sufism encourages one to be highly critical of everything they are told, and to believe little that they themselves cannot personally verify.