The animals at Manor Farm were being mistreated by their master, Mr. Jones. So the pig who led the animals, Old Major, convinced the rest of the animals to rebel against the master by teaching them a song called "Beasts of England". He then died.
It's all Snowball's fault
After the animals drove away Mr. Jones, two pigs named Snowball and Napolean began fighting over leadership of the Animal Farm. And while they were arguing, Mr. Jones attacked the farm to get it back but the animals won. They called it "The Battle of Cowshed".
Snowball announced his plans to build a windmill, causing Napolean to bring nine big dogs to chase Snowball away and to attack and eat any animals that didn't agree with him. Napolean was then the dictator of the farm.
Things began going downhill and Napolean blames Snowball for all the problems. He unleashed the dogs on animals he said were working with Snowball.
Things are getting worse and worse on the farm, with no signs of anything getting better. Boxer dies.
In the end, the pigs end up being no better than the humans they once tried to escape. Napolean changed the name of the farm back to Manor Farm during a game with a human. The rest of the pigs and Napolean walked on two legs like humans.