Well...it isn't hard to figure out who Orwell wanted to represent whom as their characteristics pretty much match their historical counterparts...but I'll give you my take on it.
Farmer Jones represents Czar Nicolas. He is selfish, lazy, and not too bright. Because he thinks only of himself, the animals are neglected and the rebellion begins.
Squealer is devious and a lie-teller. He'll do anything for the cause. He spins the truth to get people to believe in his cause. (Media)
Napoleon is ruthless, brutal, fat, lazy, and smart. He can force others to believe what he is doing is good for them...when he only pleases himself. (Stalin)
Snowball is smart, a little selfish, but he believes in the cause. He is more concerned with himself than with others...but he is willing to work for what he believes (Trotsky)
Boxer is dumb, strong and loyal. He does not question because he can't think of what to ask, and he believes what he is told (the communist people)
Those are some of the main characters for you. You may find others on the link.