Who are Isis?

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The advancement of Isis (the Islamic State of Iraq and the Levant) across northern Iraq has taken the world by surprise. They've taken control of Iraq's second largest city Mosul, and is said to be advancing on the capital, Baghdad. They're being described "as the most powerful and effective extreme jihadi group in the world," taking over from Al Qaeda. Who are ISIS and what are their origins? Photo: AFP/Getty Images