Most accounts I've seen trace the emergence of ISIS to the Iraqi insurgency which was known as "Al Quaeda in Iraq" during the Iraq war/US Occupation. It was originally made up of Al Quaeda militants, ex-baathists from Saddam's regime who were kicked out of the Iraqi Army after Saddam fell, and disgruntled Sunnis who were marginalized/oppressed by the newly elected Shia government in Iraq following the occupation. (e.g. violent crack downs on demonstrations, exclusion from jobs, government and military). They were the one's planting IEDs and blowing up Shia mosques, and attacking coalition forces during the Iraq war.
As near as anyone can tell, their objectives are born out of sectarian, anti-imperial power struggle using religious ideology to recruit and mobilize islamic militants from around the globe to fight jihad.
When Syria's civil war was underway, they took advantage of the power vacuums in the the north of the country to actually claim and control territory and began to call themselves the Islamic State in Iraq and the Levant. They were successfully able to seize a number of oil refineries in Syria and Iraq which made them the richest terrorist group ever to have existed. Many of these have since been bombed by the US in an effort to cut of the primary source of income.
Their stated ideology is to establish a regional caliphate governed by Sharia law. They do not recognize the democratically elected government in Iraq or Assad's Shia-allied dictatorship. Ideologically like-minded individuals from around the world provide financial and material support in addition to bodies to fight.
Last summer ISIS took control of Mosul in Iraq due in part to a demoralized, disorganized Iraqi army which does not have the kind of nationalistic cohesion needed fight what is essentially a sectarian battle. It is important to note that prior to the fall of Saddam, Iraq was ruled by the minority sunni government for 30+ years which brutally crushed dissent by the Kurds and Shia. This is the reason you see the Kurdish and Shia Militias fighting more effectively against ISIS than the official Iraqi Army. After decades of brutal totalitarian control to seal the boiling pot of sectarian grievances, a sense of national allegiance does not just spring up overnight. When it comes to violence and subjugation of one group by another, the memory of who's done what to whom can take generations to fade.