Revolutions/revolts are triggered by three consecutive turns of unrest in one social class, in a region (you can acheive this by raising taxes, destroying cultural or governing buildings or removing garrisons) .

On the first turn of unrest you will receive a "letter of demands" from the settlements comparative upper class (middle class in a republic, nobility in an absolute or constitutional monarchy) or a notice that the workers have been on strike, from the comparative lower class (lower classes in an absolute monarchy or republic and middle classes in a constitutional monarchy) on the second turn you will either get another "letter of demands" from the comparative upper class or you will have a riot in the region which usually allmost destroys one building (you can avoid damage to buildings by having armies in the towns and the regional capitol however if all are occupied by an army a building will be damaged at random, use this to direct damage at cheap buildings).

On the next turn you will get a notice saying a revolt or revolution has occured, if it is the former a rebel army will appear in your region (usually five or six units) and attack the regional capitol immidiately if it is badly garrisoned or suicidally after a few turns of raiding however if your garrison is more powerful than the rebels it may still be wise to wait for them to attack as if you sally forth to fight them they may flee and take the ungarrisoned regional capitol next turn which can be irritating.

If you have upset your capitol region you will get a notice saying there has been a revolition but a regeme army has risen up to defend the capitol you then get a choice to either support the regeme or the revolutionaries, if you didnt have any armies in your capitol region when it rose up I would strongly reccomend siding with the rebels as they are allways given a more powerful army to start with and get given most of the units that you send from outside regions however if you think you can hold out long enough to recruit units from within your capitol be my guest and put down the traitors. Whichever side you choose a small battle will then occur (if you havent poured in additional units from outside regions which I wouldnt reccomend) and you will most likely win, if you have followed my tips.

Revolutions are very useful as you can change your government type which each have their own advantages and disadvantages they can allso be used to change monarch in an absolute monarchy if he/she is detrimental to your campaign. Here is a list of successful revolution outcomes.
Absolute monarchy->Nobility revolt->Absolute monarchy (purpose new monarch)
Absolute monarchy->Lower classes revolt->Republic (purpose new government type)
Constutional monarchy->Nobility revolt->Absolute monarchy (purpose new government type and new monarch)
Constutional monarchy->Middle classes revolt->Republic (purpose new government type)
Republic->Middle classes revolt->Constitutional monarchy (purpose new government type)
Republic->Lower classes revolt->Republic (no purpose)
Revolutionary armies can allso be given an excess of unique units to bolster your armies and different government types have different units available to them which you may prefer.

Revolts are nowhere near as useful as revolutions however triggering one before an enemy army takes one of your regions will slow and damage them as the rebel army retains even after they have taken the region. You may allso provoke a revolt to trigger the creation of a new faction or recreation of an old one that has been destroyed though sacraficing a region to do this is costly the rewards can be equally as good for instance trade partners late in the game are hard to come by and factions on the hard/very hard difficulty can produce many more armies than you per region should you allie to the new/recreated nation and they are usually eager to make peace with you in the beggining as they are so weak. You can allso use this tactic to create "buffer zones" from enemy frontiers though it is again very costly.

Absolute monarchy
The two classes in an absolute monarchy are the lower classes and nobility. Most factions begin as absolute monarchy's, they are an easy to manage government system as you can fire and replace ministers out of hand meaning you usually have a good parliment and a pesky monarch can soon be replaced via a nobles revolution and you will still remain an absolute monarchy. However that is where the advantages end in an absolute monarchy you are allways struggling to keep the lower classes in order this is only worsened over time by industrialisation and "clamour for reform" from research into philosophy, enlightenment and economics making absolute monarchy increasingly tiresome into the later turns, however you can outbalance this with careful choice of ministers so dont completely rule this out and the nobilty are a doddle to look after. Absolute monarchys have a small research rate penalty.

Constitutional monarchy
The two classes in a constitutional monarchy are the nobilty and the middle classes. Your monarch is more difficult to replace in a constitutional monarchy as to change or be rid of him you must change government type. You can fire one member of the cabinet each turn in a constitutional monarchy to be replaced by a random minister who could be better or worse so typically your cabinet is mediocre and there is an election every ten turns but this can be easily rigged by lowering taxes and your cabinet can be re-elected indefinately so you can maintain a good cabinet. Order wise an constitutional monarchy is very easy to look after with reduced "clamour for reform" and no industrialisation as that only effects lower classes however the final advancement "Abolition of slavery" has little advantages as you have no lower classes to please and you have a nobility to annoy.

Republic
The two classes in a republic are the middle and lower classes. Republics have a constantly changing president, pime minister, premier etc in place of a monarch which can be irritating as you have little stabilty. You may fire one minister in your cabinet each turn to be replaced randomly. Elections take place every eight turns in a republic and one party can only last two elections which gives massive instability and usually leaves you with a bad government. Industrialisation is still a problem in a republic but further reduced "clamour for reform" gives you a generally happy lower class and the "abolition of slavery" should make them tame as sheep once you get that far in the game. Republics have a small research rate bonus.

Flags are different for republics and monarchies, both kinds of monarchy share the same flag. Revolutions do not allways change the flag they only change it if you go to republic from monarchy or vice versa. Here is a list of flags for all playable factions.

Britain, Monarchy, Union flag/jack, Republic, Alternate union flag (failed earlier flag cast out because it prioritised england over scotland)
France, Monarchy, Fluer delise flag, Republic, modern french flag
Spain, Monarchy, Modern flag with royal emblem, Republic, same slightly discoloured no royal emblem
United provinces, Monarchy, Six striped fictional flag same red white and blue, Republic, Modern dutch flag
Austria, Monarchy, Double headed eagle flag with black and yellow stripes, Republic, Striped burgundy and white flag
Prussia, Monarchy, White and black flag with eagle, Republic, Modern german flag
Russia, Monarchy, Imperial russian flag like the modern flag only with the royal emblem, Republic, modern russian flag
Ottoman empire, Monarchy, Red and white flag three cresents and a star, Republic, same colours one cresent and a star
United states, Monarchy, Fictional flag with a crown instead of stars same stripes, Republic, the stars and stripes flag earlier with fewer states thus fewer stars in a circle
Poland lithuania, Monarchy, Red white red striped flag with royal emblem, Republic, Half red half white flag no emblem
Maratha confederacy, Monarchy, Orange flag with spoked wheel design in centre, Republic, modern indian flag
Sweden, Monarchy, Modern swedish flag, Republic, fictional inverted swedish flag in one corner with lighter blue backround