Sufism is not a belief system.
Sufism is the mystical element in Islam, wherein the practitioner generally meditates or repeats the Divine name. As a result of practice and focussing the mind on the divine the Sufi experiences fana, the presence of the Divine, in this case, called Allah.
Classical Sufi scholars have defined Sufism as "a science whose objective is the reparation of the heart and turning it away from all else but God"
While all Muslims believe that they are on the pathway to God and will become close to God in Paradise — after death and after the "Final Judgment" — Sufis also believe that it is possible to draw closer to God and to more fully embrace the Divine Presence in this life.