This novel is an allegory for the Russian Revolution. The aim of the revolution was to replace the oppressive czar with a government run by the people. This was the promise of communism. It ultimately failed because the concentration of power at the top became too great and too oppressive on those below.
In the novel, Old Major establishes the need for revolution. When he dies, Napoleon and Snowball intend to continue the rebellion. They will rise up and oust their owner (czar), Mr. Jones. When they do so, they gain control of the farm and rename it "Animal Farm." For a time, their working conditions, food rations, and general way of life are all much better than when they were under the oppressive Mr. Jones. But the pigs, who are the leaders, begin to take liberties and start utilizing more and more power over the other animals. Napoleon has Snowball banished from the farm and this gives him even more power and more opportunity to assert his authority. His uses Squealer to spread his propaganda and this includes occasionally changing the commandments to adapt to the pigs' growing power. By the end of the novel, the pigs (namely Napoleon) have become so powerful and oppressive that the lives of the other animals are just as bad as they were under Mr. Jones. Thus, they have come full circle, right back to where they began. The animals were oppressed by Mr. Jones and now they are equally oppressed by Napoleon. The similarity between the pigs and the humans is striking. Orwell ends the novel with this image:
The creatures outside looked from pig to man, and from man to pig, and from pig to man again; but already it was impossible to say which was which.
In the beginning, the animals revolted against the treatment of the humans. They deplored the use of whips and the long hours of labor they were forced to endure under the reign of humans. By the end of the book, the pigs have replaced the humans by way of their hideous mistreatment of the animals, including the return of the whips, forced servitude, and even the physical resemblance of pigs to humans.