Snowball is the organizing force behind the beginnings of Animal Farm. He arranges the committees, assists with the development and explanation of Animalism, and is in charge of planning the military strategy to defend the farm against Jones and the invading humans. Snowball represented Trotsky, an originator of the Russian Revolution who was driven out of Russia and into exile.
Squealer is the fast-talker of the pigs, the one who publicizes all the propaganda used to rationalize the changes in conditions and in the treatments of different animals that occur as time goes by.
You do not imagine, I hope, that we pigs are doing this in a spirit of selfishness and privilege? Many of us actually dislike milk and apples. I dislike them myself. Our sole object in taking these things is to preserve our health. Milk and apples (this has been proved by Science, comrades) contain substances absolutely necessary to the well-being of a pig.
Major is the Lenin equivalent - the intellectual, the one who supports the revolution and encourages the others to do likewise, teaching them "Beasts of England" as a rallying cry. Lenin was the theorist, the developer of the political philosophy of socialist communism that served as the basis of the government after the Revolution.
Napoleon and Stalin act as absolute dictators in the later stages, after those who were originally involved but became excess were vanquished. Both had many of their followers killed for trumped up reasons alleging betrayal to the cause; both encouraged the development of multiple titles to proclaim their greatness.
Napoleon was now never spoken of simply as "Napoleon." He was always referred to in formal style as "our Leader, Comrade Napoleon," and this pigs liked to invent for him such titles as Father of All Animals, Terror of Mankind, Protector of the Sheep-fold, Ducklings' Friend, and the like.