From what I know, they are Kharjiites, followers of the people who rebelled against Imam Ali AS during the battle of Siffin. They hold the belief that it is their right and duty to revolt/rebel against any ruler who deviate from their own interpretation of Islam.
It started initially as a khawaariji group but I think over the time they have merged together with the general Muslim community. Of course they have their own fiqh and mutazilite aqeeda, but they are one of the most peaceful Muslim sects today.
It's not a religion, just a different sect within Islam. They have some weird beliefs though, unlike Sunnis who believe everybody including believers will spend some time in hell to atone for their sins before being transferred to heaven, the ibadis believe that once you're in hell you're they're for good even if you're a believer.
* Allah swt cannot be seen , even when one is in Paradise.
* Quran is NOT speech but creation of Allah swt.
* They Take Quran to be symbolic and not literally in some verses.
* They do not belief in Khilafat and Imamat ( Do not deem it necessary ).
* They believe the Khilafat of Abu Bakr and Umar to be correct , and also first part of Uthman to be right but latter half as corrupt .
* The believe the first part of Ali rule as right but disagree with him on his making peace with Muawiyah . They believe Muawiyah and Aisha rebelled and Ali should not have signed treaty with Muawiyah.