Stormcloaks are a religious, isolationist kingdom that wants to shirk their obligations to the empire mostly just because it is going through a rough patch
The Empire is a pluralistic and multicultural empire which is basically what the Roman Empire would be if it lived up to its best values, but because of their weak response to the Oblivion Crisis + the rise of the Thalmor, they have been heavily reduced in territory to Cyrodil, Skyrim (for now), and High Rock, with an informal alliance with Hammerfell.
The Thalmor are an outright genocidal supremacist religious extremist group. The Aldmeria religion quite literally says that the high elves are basically descendants of the gods trapped to live in the material realm because of the trickery of Lorkhan, and that humans are by extension basically demons who came out of Lorkhans trickery. As such they basically see humans as less than people and totally disposable, and the only reason they aren't doing mass-genocide of humans is because the Empire has still somewhat stopped them. In the meantime though they've taken to genociding their "fellow" elven brothers in the Wood Elves because they think they have deviated from the divine form by... mutating over the several thousand years they have lived on their continent. Oh, did we mention that Ulfric was in the pockets of the Thalmor, and thus is probably literally a plant and the whole civil war organized to weaken the empire for the upcoming round 2?
tl;dr, the Thalmor are evil, the Stormcloaks are retarded, the Empire is imperfect but the best choice in the situation.