George Orwell's novel Animal Farm is an allegory about the Russian Revolution and the evils of communism. The animals are like the Russian workers and Farmer Jones is representative of the ruling class of Russia, specifically Czar Nicholas II, who was overthrown by the communists in 1917. The aging boar, Old Major, is the German philosopher Karl Marx who recounted the exploitation of the working class and the revolution that promised to cure their ills in his book The Communist Manifesto.
In his speech at the beginning of the book Old Major explains that while the animals do all the work the farmer is the one who benefits. The cows give milk, the chickens lay eggs, and the horses plow the fields, yet their lives are still miserable. According to Old Major only one thing will change the animals' plight and that is rebellion. By overthrowing the farmer, Old Major promises, the animals' problems will be solved and they will be both free and rich.
Unfortunately the revolution never quite delivers the freedom the animals want. The pigs, led by Napoleon and Snowball, eventually become similar in their repression to Farmer Jones. Napoleon is the Bolshevik leader Lenin, and Snowball, who is ousted from the farm, is Leon Trotsky, who was exiled from Russia and later murdered in Mexico. Squealer represents the communist propaganda machine and the dogs are representative of the communist secret police, the KGB.