IS claims to have fighters from the UK, France, Germany and other European countries, as well as the US, the Arab world and the Caucasus.
Unlike other rebel groups in Syria, IS is seen to be working towards an Islamic emirate that straddles Syria and Iraq.
The group has seen considerable military success. In March 2013, it took over the Syrian city of Raqqa - the first provincial capital to fall under rebel control.
In January 2014, it capitalised on growing tension between Iraq's Sunni minority and Shia-led government by taking control of the predominantly Sunni city of Fallujah, in the western province of Anbar.
It also seized large sections of the provincial capital, Ramadi, and has a presence in a number of towns near the Turkish and Syrian borders.
The group has gained a reputation for brutal rule in the areas that it controls.
However, it was its conquest of Mosul in June that sent shockwaves around the world.
