Like any good satire or allegory, and almost all of George Orwell's work, Animal Farm is full of social commentary. Though he dismissed the book as a "fairy story," Orwell wanted to show how even those popular movements that started with good intentions could become corrupted once they gained power. The pigs launch a revolution in the name of Animalism, an ideology that promotes equality to all animals and the end to exploitation by Man. We see at the beginning of the book that this set of ideas, based on a speech given by Old Major in Chapter One, is sincere and motivated by an honest desire to make a better world (or farm.) But once Jones is driven from the farm, the pigs assume leadership, and begin to use their power to enrich themselves and, most importantly, to pursue more power. The idealism of the early days of Animal Farm, exemplified by the singing of the revolutionary song "Beasts of England" and the Seven Commandments on the barn, quickly becomes corrupted as Napoleon consolidates his power through propaganda and violence. By the end of the book, the pigs are indistinguishable from the humans they overthrew, and the lives of the rest of the animals are, if anything, worse than they were before. Throughout the book, we see serious social commentary. The ability of the state to manipulate information is symbolized by Squealer. The ease with which "the masses" can be swayed by jingoism is represented by the sheep, who drown out dissent by bleating "four legs good, two legs bad" at public meetings. There are countless other examples--every character in the book is intended to say something about society.
Overall, the social message of this book can be interpreted in a number of different ways. You could argue that Orwell was simply denouncing all popular movements, which were doomed to fail because people (symbolized by animals in the book) are inherently corrupt. But Orwell was a democratic socialist for most of his adult life, including when he wrote Animal Farm. It can also be read as a fairly straightforward allegory for the emergence of brutal Stalinism from the communist revolution in Russia. There is no doubt that Orwell had the Soviet Union and the rise of Stalin in mind, and that Animal Farm is a critique of totalitarianism. But, especially when read in tandem with 1984, his other great dystopian novel, Animal Farm has an even more sobering warning: Unless we are vigilant in keeping human rights and liberties at the forefront of our politics, all of our political systems are subject to sliding into totalitarianism.