Sunnis are more of the orthodox Islam. Sufis are the MYSTICAL ORDER of Islam, though technically they did not come from Islam but from other mystical orders. They work mostly with DIVINE LIGHT AND SOUND, not prayer. Their main spiritual exercise is the HU Song. Just close your eyes, look to the blank screen of the mind. You might put a spiritual image there or just let stay blank. Then, chant or sing HU (pron. Hue) and listen for the sound and look for the light. You might even imagine heavenly worlds, and be surprised when you find yourself actually being there. It's the HIGHER PATH of most religions, which has been mostly lost, yet now, is making a comeback. The Christians had Allalluah, which was Allah-HU.
Sunnis are generally an offshoot of the Orthodox version of Islam and strictly believe in the immutability of beliefs and the word of Quran and follow it to letter and spirit. Sunni Muslims do not believe in incorporating practices in Islam that were not observed by the Prophet and the original believers. However, later on when Islam and Muslims were exposed to other ideologies from the world new sects arose from those encounters one of which was the 'Batili' sect or the 'asharis' which incorporated ideas from different religions and preached that the words of Quran harbored deeper hidden meanings besides the obvious. Later on, the Sufi movement in Islam took root with Imam Ghazali who sanctioned alternate practices of the Sufis which were considered alien to the Islamic traditions before him. There are many different stories and ideas about the origins of the word "sufi", however they are people who believed in a form of spirituality and tried to incorporate it with the mainstream Islam, they had derived their ideologies from many different religious practices like the Gnostics, Aristotlean and Platonic ideas, Buddhists and other spiritualist traditions. Sufis focused more on practicing love for God rather than fearing him and all their central philosophy revolves around love as the only ambition that can assuage man from his fears and help him achieve salvation.