Mr. Jones is a fairly normal farmer, who got trapped between the normal ways of a farming community, and the ideologies of Manor Farm's animals. Mr. Jones treated his animals with the amount of respect that could have been expected by most people, and he was driven off his farm because of it. He wasn't a bad man, just a man who was in the wrong place at the wrong time.
Napoleon wanted Animal farm to be ruled as a totalitarian state with himself as the leader, instead of ruling it as an equal partner with Snowball. He tricked the animals of the farm into believing that he wanted every animal to be equal, while underneath his kind appearance, he was actually brewing up a devilish plan to take over Animal Farm as dictator. The outcome of his plan made the farm no better than it had been previously, but he only changed the ruler.
Snowball was the kinder, gentler side of Animal Farm's governing body, who intended to make Animal Farm a better place for each and every individual to live, and have all animals live in harmony. This plan was cut short by comrade Napoleon, who used his dogs to banish Snowball forever, therefor ridding Animal Farm of their kind-hearted ruler.
Squealer was just another pig until he decided to follow close behind their new ruler, Napoleon, and relay the messages of his tyranny to the other animals. He was the one who made his leader always sound like the good pig, the one doing the right thing. No matter what Napoleon did that was immoral or against the Seven Commandments, Squealer always made the animals believe that he was right and they had just forgotten or misunderstood what had been said.
Old Major was the instigator of the massive rebellion at Manor Farm, soon to be Animal Farm. He was the oldest of the animals on the farm, and he was the group's leader until his unfortunate death shortly after a community meeting at which he described his dream to the rest of the animals. Old Major had a dream of animals taking over the farms--driving all humans out. He taught the animals a song called "Beasts of England", which became their anthem.
Boxer was the Hardest working animal on the farm, because he was the strongest and so dedicated to the rebellion that he would drain every ounce of strength from his body before returning the farm to a human's ownership. He worked as long as he possibly could, and his life ended miserably at the knacker's after an accident at work on the windmill.
Clover was, along with Boxer, the most faithful of the animal comrades. She was a cart-horse who did every single thing that she was told, and she attended all of the secret meetings in the barn. Her biggest disability was that she couldn't think for herself very well at all.
Mollie was one of the animals who wasn't completely convinced that the rebellion was the best thing that could have happened, but she was not very rebellious when it came to helping out. The one thing that she thought she should have been allowed to do was wear ribbons in her mane. Snowball told her that her ribbons were "the badge of slavery." Mollie acknowledged his disapproval, but later went on to run away from Animal Farm because of that rule.
Moses is a crow who sat around nearly all day and told stories of Sugarcandy Mountain, mostly because he was used to being Mr. Jones's personal pet. He was disliked by most of the animals because he hardly did any work. Napoleon hated Moses because his stories of Sugarcandy Mountain, the place where all animals would eventually go after death and where everything is wonderful and there is no work to be done, caused some of the animals to want to go there.
The black cat was just another of the lazy citizens of Animal Farm. She always seemed to mysteriously vanish when there was hard work to be done, and then suddenly reappear at meal time. This habit cause a dislike for her among many of the working animals, although there wasn't much that any one of them could do about it.
The old donkey on the farm didn't seem to have much motivation to work hard and move fast, because after all, he would be alive long after any of the other animals died, so he would be working for a very long time! Benjamin was a good friend of Boxer, and he pointed out what the truck said as his dear friend was taken away to be "treated" at the knacker's.