What is the difference between Islam and Sufism?

Tasawwuf, or Sufism, can be called the spiritual aspect or the inner state of Islam. It is a branch of knowledge just as Islamic Jurisprudence, or Fiqh, the knowledge of Hadith or Tafsir, and as each branch of knowledge has its roots within the bounds of the Qur'an and Hadith, so does Tasawwuf.

The basic foundation of Islam is held in the beliefs, and there is also a system of morality, which concerns man's relationship with other humans and creatures and is expressed in social, civil and criminal laws. Worship is the physical aspect of man's relationship with Allah, which constitutes the basics of all other systems, may it be moral, social or legal.

If the heart accepts and believes the truth, the body follows it in worship in proportion to the extent of the faith. If the inner aspect of faith becomes weak, the outward form of worship becomes soulless. It is the human's direct relationship with his Creator which is the breath and life of religion, and this relationship is cultivated through Tasawwuf. [Keller, Sea without Shore]

The religion is made up of the three fundamentals mentioned in the hadith in which the angel Jibril peace be upon him, known also as Gabriel, questioned the Prophet ﷺ. Of these, Islam is the Shariah and its ancillary disciplines, and the level of ihsan is Tasawwuf.

Many Muslim scholars knew the value of Sufism as an ancillary Shariah discipline needed to purify the heart, and through the ages of Islamic civilization it was recognized as the science of ikhlas, or sincerity, needed by every Muslim, as the Qur'an mentions: "a day when wealth will not avail, nor sons, but only him who brings Allah a sound heart" [Qur'an, ash-Shu'ara 26:88].