A revolt occurs when the peasants and commoners rise up against their lawful lords and begins assaulting the holdings in a province.
Revolt risk rises in provinces that are of a different religion or culture than their owner. It rises even more if they're not only a different culture but a different culture group (and likewise not only a different religion but a different religious group). There are also events that can raise revolt risk.
Pre-The Old Gods
The following describes the old system, which is in effect without The Old Gods DLC.
When the check against revolt risk passes, a stack of rebels spawns in the county consisting of light infantry, archers, and usually cavalry. It is 1.25x the size of the largest garrison in the province and is hostile to everyone. If the rebels manage to conquer a province, then events can occur which can destroy buildings or wound or kill a local landholder. A province occupied long enough can even break away or join another realm.
Usually, if the rebels spawn in the lands of a vassal duke with multiple holdings, he will muster his own levies and retinue to deal with the rebels, but fundamentally it is the duty of a liege-lord to handle rebels, and frequently your count-level vassals won't be able to deal with rebel stacks. Rebels usually disperse once they lose one battle, obviating the need to chase them around the map.
The Old Gods
With 
The Old Gods active, rebels come in four types: peasant revolts, religious revolts, heretic revolts and liberation revolts. The rebellion has a leader with a duchy-tier temporary title, who declares war on the realm but is also hostile against all other independent rulers.
All rebellion events trigger on the first day of the month. The rebellion spawns in a county with an army scaled to the size of its top liege's levies, in the same manner as adventurers. If there is an ongoing rebellion with the same goal, additional troops join the existing rebellion rather than declaring a duplicate war. If you have 100% warscore (usually due to wiping out the enemy army), the war ends.
The rebellion has the same effects as any war (such as interrupting feasts and other event chains that require you to be at peace and preventing anyone from usurping your titles). If it wins, the revolt title is destroyed and the leader becomes independent. If it loses, the leader is imprisoned by the defender and gains the "Broken Spirit" modifier, making them useless as future leaders.
While revolt risk depends on the difference between the province and the local ruler, the CB used by rebels depends on the difference between the province and the top liege:
- Peasant revolt
- A peasant revolt is led by a randomly generated lowborn character of the county's religion and culture with the
trait. The title and CB are named "[County adjective] Peasant Revolt" (e.g. "Cornish Peasant Revolt"). On success, only the target county is usurped. - Religious uprising
- If the county's religion is a different religious group from the top ruler, the revolt may be a religious uprising, led by a randomly generated character of the county's religion and culture. The title and CB are named "[County adjective] [Religion] Uprising" (e.g. "Syrian Orthodox Uprising"). On success, the attack usurps all occupied holdings (along with minor holdings in occupied counties) and holdings in the targeted duchy (unless held by characters of the correct religion who can be vassalized instead). The attacker automatically usurps the duchy title if it is controlled by the defender or does not exist.
- Heretic uprising
- Similar to religious uprisings, triggered when the province religion is a heresy of the top liege's religion. Led by a leader with the
trait. - Liberation revolt
- A more dangerous kind of revolt, this targets an entire de jure kingdom whose title does not exist but did in the past (possibly earlier than the game's start date). This type of revolt can happen when the province culture differs from the top liege and the "kingdom culture" (that is, the traditional culture of the de jure kingdom in question) matches the province culture. The titular title is "[Kingdom] Liberation Revolt" (e.g. "Welsh Liberation Revolt"). If the rebels win, they gain occupied holdings along with all territory in the target kingdom controlled by the liege. If the resulting realm has 5 or more counties, the newly independent ruler creates or usurps the kingdom title (unless it is now held by an uninvolved ruler).
Revolt risk
Each province has a revolt risk that determines the chance of a revolt breaking out.
Due to differences between the province population and the local ruler (or regent):
- +1%: Different culture (of same culture group)
- +2%: Different culture group
- +1%: Different religion (of same religion group)
- +2%: Different religion group
- +3%: Heresy
Due to other circumstances:
- +4%: Isolated County (not the holder's capital, and not adjacent to another demesne province)
- +4%: Nomad agitation in provinces taken from
nomadic realms
Due to temporary province modifiers, triggered by events and shown as red icons:
- +5%: Incompetent ruler (caused by a ruler being in debt)
- +5%: High taxes (caused by Collect Taxes mission)
- +5%: Triple Taxation! (caused by Extort Peasants with the Build War Chest ambition)
- +10%: Cultural unrest
- +10%: Religious tension (can be triggered during Proselytize mission, or by low moral authority in Indian religions)
- +50%: Religious unrest
- +50%: Heretic stronghold (province where a heresy appeared spontaneously)
- Various crises during epidemics, with
The Reaper's Due
Strategies
- Lower revolt risk:
- Spread your culture, or keep vassals of the same culture as their province
- Spread your religion, or keep vassals of the same religion as their province
- Keep moral authority high to avoid having heretic strongholds appear.
- Convert counties quickly, especially if the religion of the counties have a low MA
- Crush religious revolts quickly, lest additional troops join the ongoing revolt.
- Offer white peace to rebel leaders once you have destroyed approximately half of their army. You still get to imprison the leader. Only go for full victory if you need the additional prestige or piety.
- While you cannot call allies to help you with revolts, you can call them to join you in another war, and use their attached armies against the rebels.
- If you cannot spare the soldiers to defeat an independence or peasant revolt in your de jure territory, an alternative solution is to surrender before too much damage is done. You will take a prestige hit, but if you can afford it you will immediately gain a variety of claims on the errant territory, allowing you to quickly reconquer it with your allies' support. This also severely weakens the enemy, especially in the case of peasant rebellions: they will suffer new administration penalties to levies and taxes, and much of the active army will have immediately dispersed upon their earlier "victory". And as you were the defender during the preceding war, the relevant truce only binds the rebel leader as the aggressor.