ISIS stands for Islamic State in Iraq and al-Sh?m. Some prefer to refer to al-Sham as Syria. Some see al-Sh?m as representing the 'Levant', which is a loose term for a large region in the Middle East. The makes the group ISIL. The group's full Arabic name is al-Dawla al-Islamiya fi Iraq we al-Sham, the acronym of which is DAIISH. The group doesn't like to be called DAIISH because it's phonetically very similar to another Arab word, "dahes", which means "one who sows discord".
ISIS has killed a little over 10,000 people many of whom are Muslims.