What theological and historical factors distinguish Ibadi Islam from either Shia or Sunni schools?

I'm learning much about the division of Shia and Sunni Islam from [this wonderful primer](http://www.cfr.org/peace-conflict-and-human-rights/sunni-shia-divide/p33176#!/) by the Council on Foreign Relations. I'm understanding the main divisions resulting from the succession question and understanding the importance of Karbala, etc... I'm seeing generally that Sunni's predominate, but that there is a strong Shia presence in Iraq, Iran, and Azeribaijan. And then, when I look at this map of the various schools, I'm seeing Oman. They seem to be largely out of the geopolitical debate, and from the "family tree" seem to be an offshoot that is neither Sunni nor Shia. What exactly are they? How do they differ theologically or historically from Sunni Islam?