Sufism (Arabic ta’awwuf, meaning “the state of being a Sufi”) is a mystical tradition of Islam.
There are a number of different Sufi orders that follow the teachings of particular spiritual masters, but the bond that unites all Sufis is the concept of ego annihilation (removal of the subject/object dichotomy between humankind and the divine) through various spiritual exercises and a persistent, ever-increasing longing for union with the divine. “The goal,” as Reza Aslan writes, “is to create an inseparable union between the individual and the Divine.”

A teaching comparable to the teachings of the Christian mystic St. John of the Cross.

The central doctrine of Sufism, sometimes called Wahdat-ul-Wujood or Wahdat al-Wujud or Unity of Being, is the Sufi understanding of Tawhid (the oneness of God; absolute monotheism).

TawhÄ«d is the Islamic concept of monotheism In Islam, TawhÄ«d means to assert the unity of God. The opposite of TawhÄ«d is shirk, which means “division” in Arabic, referring to idolatry. 

For Sufis, Tawhid implies that all phenomena are manifestations of a single reality, or Wujud (being), which is indeed al-Haq (Truth, God). The essence of Being/Truth/God is devoid of every form and quality, and hence unmanifest, yet it is inseparable from every form and phenomenon, either material or spiritual. It is often understood to imply that every phenomenon is an aspect of Truth and at the same time attribution of existence to it is false. The chief aim of all Sufis then is to let go of all notions of duality (and therefore of the individual self also), and realize the divine unity which is considered to be the truth.

Jalal ad-Din Muhammad Rumi, (1207-1273), one of the most famous Sufi masters and poets, has written that what humans perceive as duality is in fact a veil, masking the reality of the Oneness of existence. “All desires, preferences, affections, and loves people have for all sorts of things,” he writes, are veils. He continues: “When one passes beyond this world and sees that Sovereign (God) without these ‘veils,’ then one will realize that all those things were ‘veils’ and ‘coverings’ and that what they were seeking was in reality that One.” The veils, or rather, duality, exists for a purpose, however, Rumi contends. If God as the divine, singular essence of all existence were to be made fully manifest to us, he counsels, we would not be able to bear it and would immediately cease to exist as individuals.

http://en.wikipedia.org/wiki/Sufism