I can't find anything on the subject that isn't extremely biased in either direction... Can anyone give me a down to earth answer to what ISIS's ideology is, what exactly are they fighting for? They must have a cause, because you can't rally that many people behind a mindless killing spree.
This was covered in a great detail in a recent Atlantic article, apropos titled "What ISIS Really Wants".
Short version of the goals:
Establish a real Caliphate.
“This is a duty upon the Muslims—a duty that has been lost for centuries … The Muslims sin by losing it, and they must always seek to establish it.” (Quoting from Baghdadi's speech on the topic of Caliphate)
In general - and in conjunction of what "real" Caliphate is - fully establish the ways and laws of Shariah.
One of the biggest disagreements between Al-Quaeda and ISIS was indeed the propensity of the latter to call anyone who merely sinned takfir (apostate) - the article fully explains the distinction.
To ISIS, even Kingdom of Saudi Arabia's implementation of Sharia is uber-lax and extra-liberal.
Prepare for, and help, the "End of Days"
This is hugely important to understand both the goals, the strategy and the tactics of ISIS. Their ultimate goals isn't to survive. Their ultimate goal is to fulfill an apocalyptic prophecy of global conflict with "Rome" (the West but also apostatic Islam); followed by the rise of anti-Messiah and divine intervention to help the ISIS triumph.
As far as I can tell, Islamic State's short-term goals are to conquer as much territory and recruit as many followers as they can in order to establish a society built on their own interpretation of Islam under strict Sharia law and the rule of the Caliph.
Long term however they want to incite Western forces (the army of "Rome") to battle them and eventually they want to bring about a huge battle between themselves and the West at Dabiq in Syria, which they believe will bring about the apocalypse.
Islamic State is a group of extremist fundamentalist Muslims, many of whom are learned Islamic scholars, who are apparently willing to sacrifice everything in the name of ideological purity.
This is far from a complete answer and I recommend you do your own research (and avoid mainstream news sources), the article user4012 linked to in their answer above is not a bad place to start.
I am not sure that I agree they must have a cause. As far as I know, they do have one, sort of, and it's especially important to recruit fighters from Western Europe but civil wars easily become self-sustaining and are just as much about survival, power and resources than about any cause (cf. the wars in the east of Congo for example). If you are a young man in the area, you don't really have a choice but to join one side or the other and the same us-or-them alternative also exists for the whole organisation.
Also, it seems political or ethnic identities are very important in this case. The Wikipedia article also highlights the role of a former intelligence officer who had lost all the power and influence he had under the old Iraqi regime. I have no idea whether that man truly believed in something, just wanted to get back at those who imprisoned him or saw this as a way to play a big role again (or maybe simply to make money?) but there is much more to it than a cause.
ISIS was created by the US and every report or article which tries to root ISIS and the mayhem they cause in the teachings of Islam ignore this fact and demonise one of the three monotheistic religions, with around a billion followers worldwide.
Watch an interview with President Putin you wont find in the western media:
Look what General Clark revealed:
The people in Iraq and Syria, the Shia, Sunni, Arabs, Kurds, Christians, Yazedihs etc. lived in peace together for centuries and suddenly they decided to fight each other? Of course there have been differences between the groups but why are these differences magnified now so that they cause the destruction of whole countries? Why are the main victims of terror Muslims? Muslim kills Muslim and commits the most heinous crimes against the follower of his own religion.
Efforts to link IS Terror to the prophet of Islam or to its true teachings are redicoulos.Everything IS does is not islamic but a barbaric crime, being funded by US allies Saudi Arabia, Qatar etc.
Why did'nt and doesn't ISIS ever turn against Israel, the well known enemy of Muslims, or even say one word against them? Didn't the Prophet of Islam fight against the Jews and - when everything ISIS does is backed by the early history of Islam and the actions of the prophet,as some maintain - so why don't they fight against Israel. And why does Israel support ISIS and treat the wounded terrorists in their hospitals, when on the other side they daily kill Muslims and want take over the al-Aqsa Mosque?
As Putin stated in the video clip, the IS terrorist are mercenaries who get paid for what they do. Surely, there are people in the low hierarchy of IS, who have been told they fight in the way of God and go to paradiese when they kill their fellow human beings. They are totally misinformed about Islam, following the Wahhabi ideology, which came to power with the help of the British government.
When putting the terrorists under scrutiny, who committed the attacks in Paris, we find that they were robbers, drug and alkohol consuming, bar owners, not practicing strict Muslims, who lived according to the Qur'an and followed the teachings of the prophet.
But all western mainstream media try to connect the terrorist acts to Islam. A reporter stated that one of the Paris attackers had left his apartment at 4:30 am to go for morning prayers to the mosque. I cannot believe that, why should a drug and alkohol consumer with girlfriend wish to pray, but it's always good to mention "prayers" and "mosque" when the world is sitting in front of the TV being scared by 24 h live news reporting from the crime scene. By the way, prayer time was around 6:12 am on that day so he left nearly 2 hours early.... maybe the mosque was not around the corner.
In conclusion, ISIS is set up to destroy the image of Islam and cover up the true teachings of Islam and to destroy independent Arab countries under the well known motto of "Divide and Rule".