- Throw off the shackles of oppression! We are the disposable lackeys of Ulfric Stormcloak, greatest war criminal in a hundred years! Together, we will destroy the Empire and by doing so send Skyrim to a thousand years of chaos! We will commit genocide and make Skyrim weak! Nords arise, for Ulfric!
- —The Stormcloak creed, or something
The Stormcloaks are a bunch of xenophobes and racists led by Ulfric Stormcloak who think elves and Khajiit are inferior to Nords and need to get out of Skyrim (they're probably right, but that's neither here nor there). The Stormcloaks form one of the three main sides in the Skyrim Civil War.
The Stormcloaks had formed as early as twenty-seven minutes before the Skyrim Civil War. After the Skyrim-Loompaland War, King Torygg surrendered to the Oompa Loompas and pretty much gave them whatever they wanted. They banned the worship of some phony baloney god and allowed all of the foreigners to live equally.
In revenge, Ulfric killed King Torygg
in a fair duel brutally and without remorse. The Imperial Legion declared Ulfric to be an enemy of the state. Ulfric retaliated by gathering a bunch of pirates, warriors, and elf-haters to attack the Empire. The Skyrim Civil War had begun.
The Stormcloaks fought the Imperials for two years, never giving up their conquest to destroy the Empire and leave Skyrim a sitting duck to enemy attacks. Except for one slight problem. Combining the Stormcloaks' lack of training and the Empire's "we don't give a crap" attitude, Nobody actually accomplished anything. The war came to a standstill.
The Stormcloaks were ultimately destroyed when Sheogorath, disgusted by the lack of bloodshed, called upon his Daedric Army to kill all of the Imperials and Stormcloaks, and set up a new era of insanity and chaos in Skyrim. A small faction of Stormcloaks remains hidden in Skyrim, but they're actually just Nords that smacked their heads against the wall too hard and are now convinced they are still in the war.
The Stormcloaks believe that the Empire is "unjust" and "oppressing the Nords". This is actually crap the Ulfric made up so he could convince stupid people to give him more power. The Stormcloaks also believe that it's the elves' fault. Nobody knows what "it" is, but the Stormcloaks assure you it is in fact the elves' fault.
The Stormcloaks are also bitter over the fact that the Empire is basically brownnosing Loompaland. The Stormcloaks know that the Oompa Loompas are just short guys that like to sing a lot. Also, the Oompa Loompas are bloodthirsy animals that will kill any nord within three hundred feed (Ulfric might have made this part up, but the rumor sure has helped decrease the Oompa Loompa population).