The book ends with the pigs beginning to physically resemble the humans as they play cards in the house. The ruling pigs have actually been corrupted by the power they wield over the other animals and behave as though they are above the law—a privileged ruling class. In this way we see that the oppressed become the oppressors. Orwell’s message is that a society needs constant vigilance and meaningful participation to remain viable.
In the old animated version of the movie the ending is different. We see the animals actually stage a revolt. Once the animals realize that the pigs have become like the humans, they appear to be indignant and outraged and they take matters into their own hands. Orwell would probably think this ending makes sense, but the ending appears to happen much too easily. I think he wanted to leave his story with a more somber ending, one that served as a warning.
In the more recent movie version, Animal Farm actually falls apart at the end, unable to recover from Napoleon’s excesses, and a new owner arrives. Jessie says that the animals will do a better job of running their society this time. It’s a more optimistic ending, but one I don’t think George Orwell would have approved of.