In a nutshell, consider the following:
Squealer, Napoleon's 'right arm pig,' both knows and acts (he is intelligent and manipulative); the donkey Benjamin knows but does not act (He is smart enough to see what is going on but doesn't say anything); Boxer, the farm's workhorse, neither knows nor acts, except blindly to follow Napoleon.
These three characters embody the hierarchy which naturally fell into place in Russia after the fall of the czar. Things didn't really change that much; there was just a change of oligarchy. Instead of Nicholas there was Lenin, then Stalin (who got rid of Trotsky, portrayed by Snowball); Squealer represents the chosen elite who were simply opportunist and curried favour to solicit privilege; Benjamin represents those who realized the abuse of the state but in order to save their skins stayed quiet; Boxer represents the naïve idealists of the working class who sincerely strove to bring about a democratic state but just got exploited in the end. (There are other analogies, too, such as Bluebell's puppies as the secret police.)
Check out the following references for further study.