On Sunday, an American-born terrorist named Omar Mir Seddique Mateen killed 49 and wounded 53 in Orlando. In a 911 call during the attack Mateen pledged his allegiance to the terrorist group ISIS. Although the group also claimed responsibility for the attack, U.S. officials said they haven’t seen a direct link between the gunman and the terrorist group.

Here are five facts you should know about ISIS:

1. ISIS (aka ISIL, Islamic State, IS, Daesh) is the name of an Islamic militant group that was established in Iraq in 2004 and pledged allegiance to “Al-Qaeda in Iraq.” They later broke away fromAl-Qaeda because of differences indoctrine and objectivesand formed a distinct organization.From late 2006 to mid 2013, the group called itself the Islamic State of Iraq (ISI). From 2013 to mid 2014, when they expanded into Syria, they called themselves the Islamic State of Iraq and the Levant (ISIL). (Most Western media translate “Levant” as “Syria,” hence ISIS.) Since 2014, they have expanded their ambitions to be a global organization and today simply refer to themselves as “Islamic State.” Enemies of ISIS sometimes refer to it as Daesh, a loose acronym of the Arabic for “Islamic State of Iraq and the Levant” (al-Dawlaal-Islamiya al-Iraq al-Sham).

2. The stated long-term goal of ISIS is toestablish a “caliphate”to rule over the entire Muslim world, under a single leader and in line with Sharia (Islamic law). Acaliphateis a form of Islamic government led by a caliph, a person considered a political and religious successor to the Islamic prophet, Muhammad. ISIS has ordered that all able Muslims around the world must emigrate to the territory under its control. As the group’s leader, Abu Bakr al-Baghdadi, stated in an audio address in 2014:“O Muslims in all places. Whoso is able to emigrate to ISIS, let him emigrate. For emigration to the Abode of Islam is obligatory.”

3. The religious-political ideology of ISIS isSalafi-jihadism(sometimes referred to simply as “jihadism”), a distinct strand of militant Sunni Islamism. Salafi-jihadist groups like ISIS emphasize the importance of returning to a “pure” Islam, that of the Salaf, the pious ancestors. Such groups also maintainthat violentjihad is a personal religious duty of all Muslim believers. Former ISIS leader Abu ‘Umar al-Baghdadi once emphasized the importance of“offensive jihad,” which he defined as “going after the apostate unbelievers by attacking [them] in their home territory, in order to make God’s word most high and until there is no persecution.” (Under their doctrine, “persecution” is understood to mean idolatry.)

4. The most prominent elements ofISIS’s religious-political doctrinerequire that: all Muslims must associate exclusively with fellow “true” Muslims and dissociate from anyone not fitting this narrow definition; failure to rule in accordance with God’s law constitutes unbelief; fighting ISIS is tantamount to apostasy; all Shi‘a Muslims are apostates deserving of death; and the Muslim Brotherhood and Hamas are traitors against Islam because promise with the non-caliphate political process (e.g., democracy).

5. The focus on personal jihad makes Salafi-jihadist groups like ISIS considerably different thanmost terrorist groups throughout history. For example, in the 1970smost Marxist and pan-Arabic terrorist groups killed people mitted other acts of terrorism in order to bring attention to their cause. For Salafi-jihadists, though,killing large number of “apostates”is itself a worthy religious objective.