Sufism is the mystical or esoteric school of thought in Islam. The Arabic word for Sufism is ‘tasawuff’. A practitioner of Sufism is called a Sufi or a Dervish. The following quotations reveal the various understandings of Sufism.
“Tasawouf, or Sufism, is the esoteric school of Islam, founded on the pursuit of spiritual truth as a definite goal to attain: the truth of understanding reality as it truly is, as knowledge, and so achieving ma’arefat. In Tasawouf when we speak of understanding or cognition we refer to that perfect self-understanding that leads to the understanding of the Divine. This very logical principle is based on a typically succinct saying of Prophet Mohammed: ‘Whoever knows oneself, knows one’s Lord.'” 
“Sufism is dedication to worship, total dedication to Allah most High, disregard for the finery and ornament of the world, abstinence from the pleasure, wealth, and prestige sought by most men, and retiring from others to worship alone.” 
“Sufism is not a sect, like Shiism or Sunnism, but rather the mystical side of Islam—a personal, experiential approach to Allah, which contrasts with the prescriptive, doctrinal approach of fundamentalists like the Taliban.” 
“Sufism is a school for the actualization of divine ethics. It involves an enlightened inner being, not intellectual proof; revelation and witnessing, not logic. By divine ethics, we are referring to ethics that transcend mere social convention, a way of being that is the actualization of the attributes of God.” 
Sufies are quite popular in everyplace where Islam has a foothold. In India, there are many shrines called ‘dargas’ dedicated to Sufi saints and these are the places people from all religions come to offer prayers through the mediation of the dead Sufi saint. Thus Sufism opens the door for religious tolerance and harmony in a pluralistic society like India, where Hindu – Muslim hatred is part of history and daily life.
Sufism is relatively unknown to most non-Muslims. Since it is the fundamentalists who disrupt daily life and make the headlines, the Sufi side of Islam is almost a surprise to many Westerners who have been made to believe that Islam is all about violence and oppression. Though this picture is not entirely false as evidenced in the media daily, fundamentalism is not the only school of thought in Islam. This is important to understand, so that we do not develop any prejudice which blinds us to the whole and so that we do not develop an aversion to Muslims as a matter of principle.
Like all Muslims, Sufies believe in the Shahada, the Sharia, and the five pillars of Islam. But the distinguishing character of Sufism is the teacher-student method of learning. Sufism cannot be learnt from books alone. A teacher, for whom a student has an innate liking is necessary. This innate liking seems to be given to the student from Allah. When a teacher took in a number of students, a Tariqa or fraternity/order was formed.
Tariqa: Sufi fraternity or Sufi order. There are many orders in Sufism as in Roman Catholicism. Some Catholic orders include Franciscans, Augustinians etc. Some Sufi orders include Mevlevi, Bektakshi etc. The Sufi saint is called as ‘Pir’.
“In pursuit of this goal of worshipping Allah, Sufis belong to Tariqas, or orders, established in the first few centuries after the Prophet’s death. These orders have a master who will teach sacred knowledge to others in the group.” 
Man is essentially self-centered or ego-centered. In this state he is blinded to spiritual reality, his heart is rusted and his faculties are in a downgraded state. Man has to purify and prepare himself so that he can experience God. A mystical experience opens up his latent spiritual faculties and makes him aware of God. Sufism is a way of purifying oneself and experiencing God in a mystical way. So Sufies believe that other than the five senses, there are in humans latent senses which become activated through a mystical experience. 
“Many people attain this level of awareness at some point in their lives. An encounter, event, or realization opens them to a reality greater than themselves. For most, this level of awakening is enough. But others desire something more: to contact God, to see the Divine, to experience Truth. Being a mere part is insufficient; they long to annihilate themselves in the Whole, the Eternal. They want their faith to spring forth spontaneously and continuously, like water rushing from a fountain. They yearn to realize in a personal way that God is as near as your jugular vein. How can this yearning be fulfilled? God is the Sublime Being; humans are gross in comparison. Their senses can hear, touch, see, taste, and smell material things, but the Supreme Being eludes detection by these means. How can a particle contact the sun? How can a part become the whole? Human beings from the beginning of time have tried to resolve this dilemma.”
Fana: ‘Fanā, ʾ (“to pass away,” or “to cease to exist”),
“the complete denial of self and the realization of God that is one of the steps taken by the Muslim Ṣūfī’.” 
Shath, plural Shaṭaḥat, in Ṣūfī Islām,
” divinely inspired statements that Ṣūfīs utter in their mystical state of fana (passing away of the self). 
Hal: It is a state of mind acquired and retained purely by the grace of God. No amount of merit is of any use. The plural of hal is ahwal.
“Though the Ṣūfīs spoke of hundreds of aḥwāl, the following are among those most often referred to. (1) The ḥāl of murāqabah (“watching”) fills the Ṣūfī with either fear or joy according to the aspect of God revealed to him. (2) The ḥāl of qurb (“nearness”) is a state that enables the Ṣūfī to become unconscious of his own acts and to see God’s acts and bounties toward him. (3) The ḥāl of wajd (“ecstasy”) is a state described by the Ṣūfī as a sensation that encounters the heart and produces such varied effects as sorrow or joy, fear or love, contentment or restlessness. (4) In the ḥāl of sukr (“intoxication”) the Ṣūfī, while not totally unaware of the things that surround him, becomes half-dazed because his association with God dims his sight of other things. The overpowering sense of the beloved in this state destroys the mystic’s ability to distinguish between physical pain and pleasure. Ṣahw (“sobriety”) immediately follows sukr, but the memories of the previous experience remain vivid and become a source of immense spiritual joy. (5) The ḥāl of wudd (“intimacy”) is characterized by “the removal of nervousness, together with the persistence of awe.” The Ṣūfī becomes calm, contented, and reassured, but the overwhelming sense of the divine presence fills his heart with the kind of awe that is free from fear.” 
Tawhid Witness : Tawhid is the belief that “there is no God but Allah”. To realize this true God individually in a mystical way is the aim of the Sufi. Over time the meaning of ‘experiencing the tawhid’ changed. 
“Early Sufism postulated the approach to God through love and voluntary suffering until a unity of will was reached; Junayd spoke of “recognizing God as He was before creation”; God is seen as the One and only actor; He alone “has the right to say ‘I’.” Later, tawḥīd came to mean the knowledge that there is nothing existent but God, or the ability to see God and creation as two aspects of one reality, reflecting each other and depending upon each other (waḥdat al-wujūd).”
Dhawq: Direct mystical experience of the divine in the Islamic way.
“‘Dhawq, direct “tasting” of experience, was essential for them. But the inspirations and “unveilings” that God grants such mystics by special grace must never contradict the Qurʾān and tradition and are valid only for the person concerned. Even the Malāmatīs, who attracted public contempt upon themselves by outwardly acting against the law, in private life strictly followed the divine commands. Mystics who expressed in their poetry their disinterest in, and even contempt of, the traditional formal religions never forgot that Islam is the highest manifestation of divine wisdom.'” 
 Ibn Khaldun, 14th century Arab historian, quoted in Keller, Nuh Ha Mim, The Place of Tasawwuf in Traditional Islam, www.masud.co.uk, 1995