A  man who'd pledged allegiance to The Islamic State shot and killed 50 people at a nightclub in Orlando early Sunday.

Omar Mateen, the man authorities say opened fire on more than 300 people at a nightclub called Pulse, phoned 911 as the attack was happening, claiming support for the terror group.

We hear the terms out of the Middle East daily - ISIS, ISIL, caliphate. Their  meanings, for those in the West, can be confusing.

Here's a quick look at who ISIS is, what their name means and the origins of a "caliphate."

1. What is ISIS?

The group that was first known as al Qaeda in Iraq in 2004 morphed into an even  more fundamentalists group two years later and renamed itself ISIS. At its core, ISIS is a radical, anti-Western ultra-fundamentalist terror group whose aim is to establish an Islamic state – or caliphate -- in the region.  The group is aiming to establish the caliphate (more on that below) on a strip of land that stretches from Syria to central Iraq.

2. What is the difference between ISIS, ISIL and Daesh?

The term ISIS stands for the Islamic State of Iraq and Syria. Some, like President Obama, refer to the group as ISIL – which stands for Islamic State and the Levant. Still others use the term Daesh.

ISIS vs. ISIL

The difference between the Islamic State of Iraq and Syria (ISIS) and the Islamic State and the Levant (ISIL) is subtle, but important to the current leaders of ISIS.

The Islamic State of Iraq and Syria (ISIS) -- The original name for the terror group in Arabic was Al-Dawla Al-Islamiya fi al-Iraq wa al-Sham. The first three words (Al-Dawla Al-Islamiya fi al-Iraq) translates to the Islamic State of Iraq. al-Sham is a term that refers to Syria and the surrounding area.

The Islamic State of Iraq and the Levant (ISIL) – The Levant – a French phrase that means “land of the rising sun” – is the term to which a region around Syria is historically referred. That area includes Jordan, Palestine Lebanon, Syria and Israel.

Since the summer of 2014, ISIS dropped the “Levant” term from its official  name.

Daesh -- Daesh is an acronym for the Arabic phrase "al-Dawla al-Islamiya al-Iraq al-Sham," or, the Islamic State of Iraq and the Levant. It has been widely reported that ISIS members despise the use of the term, and many consider it a vulgarity. The word is close to two other Arabic words --  “Daes," meaning  someone who crushes something underfoot, and “Dahes” which means someone who sows discord.

ISIS members have threatened to cut out the tongue of anyone who utters the word.

3. Where is ISIS centered?

The group's main stronghold is the Syrian city of Raqqa.

4. Who is the leader of ISIS?

Iraqi Sunni cleric Abu Bakr al-Baghdadi became the leader of the terrorist group in 2010. There were reports he may have been wounded in an air strike in northern Iraq on June 9.

5. What is the Caliphate?

The caliphate is how Muslims have organized themselves since the death of the Prophet Mohammed in 632 AD. After Mohammed died some believed one of his sons-in-law, Ali, was a good choice as successor. A supporter of Ali became known as Shiaat Ali, later, a “Shia.”

Others believed the new leader should be someone who faithfully followed the Prophet’s way of life – Sunnah. They became the “Sunni.” The majority of Muslims are Sunnis.

6. What do they want to do, ultimately?

The  terror  group wants to build a caliphate that will rule the world.

 Here are some facts about caliphates

• The leader of the caliphate is known as the caliph. The word “caliph” comes from the Arabic khalifa, meaning “successor”.

• Caliphates are governed  under Islamic law, with the leadership elected according to Sunni practices.

•  The caliphate is meant to represent Muslims worldwide.

•  The caliphate was official declared on June 29, 2014 by ISIS – on  that day, the group dropped the “Levant” name from its title.

• Abu Bakr al-Baghdadi has declared war on Shia Muslims and anyone else who is not a Muslim.