What is "Sufism" (with details)?

All perfect praise be to Allaah, The Lord of the Worlds.  I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Initially, the surname Soofiyyah (Sufism) was given to the people who engaged themselves in worshipping Allaah and preferred to live an ascetic life. There is no harm in this act except calling it by this name, which is an innovated term in Islam.

Later on, Sufism became a mixture of various innovations and exaggerations. Many forbidden ideologies of philosophers found their ways into it, such as pantheism, immanentism and atheism, which assert that one becomes free from following Islam (i.e., not obligated by it) after reaching a prescribed stage. There is no doubt that contemporary Sufism contains many deviations such as exaggerating the status of Shaykhs and the pious and worshipping tombs and graves by making sacrifices therein and supplication and circumambulation. The Sufis worship the inhabitants of these by asking for their help and support, and this is major Shirk (polytheism) and

Allaah never forgives this kind of Shirk. Moreover, they adhere to many invented supplications and celebrate the birthday of the Prophet and others. Pantheism and immanentism still exist in many groups of Sufis.

Allaah Knows best.