ISIS, the Islamic State of Iraq and the Levant, is a brutal terrorist group associated with the civil war in Syria which has taken over areas in neighbouring Iraq.
ISIS is made up of Muslims from the Sunni arm of Islam, which split from the rival Shia form of the religion hundreds of years ago.
The areas it has claimed to have conquered in Iraq are largely Sunni.
ISIS is known for its harsh interpretation of Islam and brutal violence directed at Shia Muslims and Christians in particular.
It claims to have at least 4,000 fighters in its ranks, who, in addition to attacks on government and military targets, have claimed responsibility for attacks that have killed thousands of civilians.
ISIS had close links with al-Qaeda until 2014, but in February, after an eight-month power struggle, al-Qaeda cut all ties with the group, citing, among other reasons, its "wanton brutality".
The extremists of ISIS have boasted of beheading captured soldiers and civilians who are against it.
It has proclaimed a "caliphate", or strict Muslim state, in its zone of influence in Syria and Iraq. It has expressed the desire to bring much of the Muslim-inhabited regions of the world under its direct political control, beginning with nearby territory in the Levant region, which includes Jordan, Israel, Palestine, Lebanon, Kuwait, Cyprus and an area in southern Turkey.
ISIS grew significantly as an organisation owing to its participation in the Syrian Civil War and the strength of its leader, Abu Bakr al-Baghdadi. Economic and political discrimination against Iraqi Sunnis since the fall of Saddam Hussein also helped it to gain support.
A Birmingham MP has said that over 1,500 young Britons have gone to fight in Iraq and Syria after being recruited by ISIS.
The number suggested by Khalid Mahmood MP is over three times the figure Foreign Secretary William Hague previously claimed.
Mahmood's warning came as British security experts claimed that the UK may suffer from the Syrian and Iraqi conflicts for "many years" in the future.
He also said that young people were more likely to get indoctrinated in universities and prisons than mosques.
Yesterday, the Home Secretary said British Muslim families should act if they suspect their children or siblings are thinking of travelling to join conflicts in Syria and Iraq.
Theresa May has backed a nationwide campaign by not-for-profit organisation Families Against Stress and Trauma (Fast), which puts the emphasis on families preventing their loved ones from travelling to join the conflicts.
Speaking in London, she said: "I would encourage all families to see the answer lying with them and their communities."
Meanwhile, the families of three Britons who left to fight with jihadis in Syria have appeared in a government-backed video aimed at dissuading young Muslims from travelling to the Middle East.
The father of two brothers from Cardiff who joined Isis and the brother of a man suspected of carrying out a suicide bombing in Syria, appear in the film to describe the pain their decisions have caused to the men's families left behind in the UK.
Ahmed Muthana, father of Aseel Muthana, 17 and Nasser Muthana, 20, who last month appeared in an Isis recruitment video – said that he has been "devastated" and "sad" since they left for Syria.
The father of a 41-year-old man from Crawley in West Sussex, also spoke out.
