WHAT IS TASAWWUF
Tasawwuf is the name of two qualities: First [quality] is that [the slave] should be obedient to Allah, i.e., he/she should be acting on the Shari’ah (the outer laws of Islam including obligations, commandments, and prohibitions). And second [quality] is that [the seeker] is the one who is sympathetic and a doer of good towards [all of] Allah’s creations [which is not possible without purifying one’s soul from all evil qualities and then embellishing one’s soul with all good qualities]. One who has perseverance in acting on the Shari’ah and has the qualities of [desiring] betterment (true success) for the entire humanity is a “Sufi” (this actually is the correct use of this term as opposed to the wrong uses of the term that are prevalent today). Loyalty [and obedience] to Allah is that [the seeker] sacrifices his own happiness for the sake of Allah. Sincerity to [all] people [of the world] is that [the seeker] does not keep relations with the people for his/her own motives [but instead is selfless towards them and performs selfless duty to them] and refrains from selfishness. But instead should dedicate himself/herself for the good (welfare) of [all] the people [of the world], however the condition is that, this welfare should be in accordance with the Shari’ah (the Divine laws).
Imam Ghazali (RA) To One Of His Disciple