What Is Sufism and Why Does It Bother Some Muslims?

When a mosque in Egypt's Sinai region was attacked by affiliates of the Islamic State armed group in November leaving over 300 people dead the attackers said they were targeting what they described as 'heretics of Islam', known to the wider world as Sufis.Who are the Sufis and why have they been singled-out by some other Muslims? 

The term ‘Sufi', will, for many, conjur up images of poets like the Persian Jal?l ad-D?n Muhammad R?m? from the 13th century, or the Ab? 'Abdill?h Mu?ammad ibn 'Al? ibn Mu?ammad ibn `Arab? from Andalusia in Spain from the 12th century, or even Turkey's whirling dervishes.

While poetry and dervishes can be part of different Sufi orders, at the heart of Sufism, is Islam, and its interaction with the prophet Mohammed.