Who are the Yazidi?

ISIS has attacked the Yazidi people - a sect of the Kurds - in Iraq. The jihadists have driven the survivors to the slopes of Mt. Sinjar, where the Yazidi are running out of food, water and hope, while the ISIS militants advance to wipe them out in what would be a genocide. The White House has just authorized air drops and airstrikes to save them and the U.S. and France have committed to helping the Kurds.

Who are the Yazidi? Why is ISIS after them, why have Sunni and Shi'a attacked them in the past, and why are they at risk of genocide now?

Yazidi on Mt. Sinjar, Iraqi/Syrian Border - 1920's

Historically, the Yazidis are a religious minority of the Kurds. They are said to have existed since 2000 BCE. Estimates of the number of Yazidis vary between 100,000 and 800,000, the latter being the claim of their website. According to the same site, Yazidi refugees in Germany number 30,000.

Researchers believe that the Yazidi religion has elements from Zoroastrianism, Manicheism, Judaism, Christianity, and Islam. The Yazidis call themselves Dasin, not Yazidi. The term 'Yazidism' comes from the Persian word 'īzed', 'angel'. The name Yazidism is connected to the 6th caliph, Yazid (680-83), who, from Shi'a point of view, is one of world history's most hated men, and is highly disliked by most Sunnis, as well. There is little evidence to show what role, if any, Yazid m played in the founding or development of Yazidism.

The Yazidis don't call themselves Yazidis and they're not attached to the 6th Caliph. That hasn't stopped the Sunni and Shi'a from hating them for the name they don't call themselves, as well as for the rumors that they worship the devil.

What are their true beliefs?

In the Yazidi - or more correctly, Dasin - world view, God created the world, which is now in the care of a Heptad of seven Holy Beings, often known as Angels or heft sirr (the Seven Mysteries). Preeminent among these is Melek Taus (Tawûsê Melek in Kurdish), the Peacock Angel, who is equated with Satan or Devil by some Muslims and Christians because of a similarity in a name.
"The reason for the Yazidis reputation of being devil worshipers, is connected to the other name of Melek Taus, Shaytan, the same name as the Koran's for Satan."
According to the Kurdish linguist, Jamal Nebez, the word Taus is most probably derived from the Greek and is related to the words Zeus and Theos, alluding to the meaning of God.
In the Yazidi religion, God stands above all, but only as a creator, not as a current force. Divine power is represented by Shaykh Adii, the benevolent deity and Malak Ta'us, the peacock angel who once fell into disgrace, but then repented with seven jars of tears collected over 7,000 years that were used to extinguish the fires of hell.

There are six minor deities. The two listed above are the focus of their theology, which looks to heaven, but no longer believes in hell, as it is seen as a uneventful place since the fires were put out.
So, not only no devil worship, they don't have a hell.

The Yazidi culture has its problems in the treatment of women, honor killing, and prohibition of intermarriage. This became public after the stoning death of a Yazidi teen when she converted to Sunni Muslim to marry outside her faith.

But it's not the honor killing that has led to ISIS's horrific violence toward the Yazidi, nor has it stopped others from using the misunderstanding of their religion to both isolate them political advantage as when, during the regime of Saddam Hussein, Yazidis were considered to be Arabs and maneuvered to oppose the Kurds, in order to tilt the ethnic balance in northern Iraq.

40,000 Iraqis stranded on mountain as ISIS jihadists threaten death. Members of minority Yazidi sect face slaughter if they go down and dehydration if they stay, while 130,000 fled to Kurdish north.
The Yazidi's unique identity, despite being ethnically Kurdish, was used by the Baathist regime to isolate one from the other during Saddam's reign. Both groups changed that when the Kurds became semi-autonomous and they fought together against Baathist troops in joint Peshmerga units.
Since the 2003, the Kurds have recognized the Yazidi as ethnic Kurds though that didn't always help the Yazidi:
Vying for a Voice, Tribe in N. Iraq Feels Let Down: KHARSI, Iraq -- When the 101st Airborne first reached this remote village in Iraq's northwestern Sinjar Mountains in 2003, elderly Yazidi tribesmen were thrilled: Their ancient religious prophesy had come true. 
"We believed that Jesus Christ was coming with a force from overseas to save us," said the village leader, Khalil Sadoon Haji Jundu, wrapping his gold-trimmed cloak around him against the morning chill. [snip]. But more than two years later, as the Yazidis struggle for a political voice and an escape from the poverty they suffered during decades of oppression under President Saddam Hussein, tribesmen such as Jundu say they feel let down.

The Yazidi were concentrated around the town of Bashika. ISIS has moved in on that area. The Yazidis that didn't make it behind Kurdish lines are stuck and dying on their historical refuge on Mt. Sinjar where they're without food, water or help.

The Iraqi Kurds know the Yazidi faith is not devil worship. ISIS wants to wipe out the Yazidi in what can only be called genocide. They also want to draw out the Kurds to fight on ground to ISIS's advantage and to draw in the rest of the world for their holy war.

Forty Yazidi children are reported to have died already on the mountain. This is a crisis whose response will need to be decided in hours, not days. The White House is weighing air drops of food and water, which would save some lives, and air strikes to keep ISIS off the mountain.

ISIS are jihadists, they are terrorists, they shouldn't be underestimated. They have the initial goal of a caliphate from Iraq to Palestine and Turkey. The Arab League is, rightfully, terrified of them. The rest of the world has been trying to figure out how to deal with this new enemy that will not be ignored. With the Yazidi, they've come up with a way to make it impossible for the world to ignore them. France has noticed, as has the U.S. Others will follow.

We're not done with Iraq yet.