Age of rebellion is the second of the FFG star wars rpg’s the bulk of the book is the same as Edge of the empire, so I will be talking about the differences here. the biggest difference is that instead of Obligation this game has Duty. unlike Obligation you want a high Duty as this reflects your standing in the rebellion. the higher it goes the more equipment, supplies and respect you get from the rebellion. there are rules on how to combine the games in regard to duty and obligation. the rules for combining are pretty simple you either pick one or the other, pick both, or have some players use one or the other.
now even though the game defaults to the player being part of the rebellion. you could conceivably be part of another military group such as an independent planet or the Empire. the default time period in all three books is after episode 4 and before episode 5, though you could have your game take place in any era.
the classes and races do differ from edge of the empire, the classes and races are 100% compatible with the other two games. this game also has a lot more capital ships in it. this makes sense as unlike Edge you will actually need to fight against captial ships at some point, while in edge you run from them.
the force is again touched upon in the book, in edge the force using class/specialization mirrored more of an Obi wan type exile this force specialization mirrors Luke around episode 4 and 5 pre yoda training.
over all this is my second favorite of the three books as it gives more structure to a party so you do not suffer the whole what do we do now issue that the free roam sandbox of Edge can cause.