The theme of Animal Farm is not difficult to understand.  Orwell intended to criticize the communist regime he saw sweeping through Russia and spreading to Europe and even the United States.  Though he agreed with many Marxist principles, Orwell was unable to accept the communist interpretation of socialism because he saw many similarities between the communist governments and the previous czarist regimes in old Russia.  Communism, he thought, was inherently hypocritical.

In his self-proclaimed "fairy-story," Orwell uses his allegorical farm to symbolize the communist system.  Though the original intention of overthrowing Mr. Jones (who represents the Czars), is not inherently evil in itself, Napoleon's subsequent adoption of nearly all of Mr. Jones' principles and harsh mistreatment of the animals proves to the reader that indeed communism is not equality, but just another form of inequality.  The pigs and dogs take most of the power for themselves, thinking that they are the best administrators of government.  Eventually the power corrupts them, and they turn on their fellow animals, eliminating competitors through propaganda and bloodshed.  This is of course a reference to Stalin, who murdered many of his own people in order to maintain his dictatorship of Russia. 

Thanks in part to Animal Farm, much of the Western world finally realized the danger of communism.  Soon a Cold War began between the world's greatest superpowers- the Soviet Union and the United States.  In the end, America would prove that capitalism and democracy could outlive a system of government-mandated equality.