One theme to Animal Farm is that power tends to corrupt those who wield it, especially if it is unchecked. We see this in the form of Napoleon, who is meant to represent the historical figure of Joseph Stalin. Napoleon, like Stalin, is able to dominate others to serve his own ends. The pigs, because of their superior intelligence, can take advantage of the other animals, and do.
Another theme is that revolutionary leaders will eventually come to resemble the leaders they sought to replace. Rulers, especially authoritarian ones, use similar techniques to keep their subjects in line, including the display of force (the dogs) and misinformation campaigns (Squealer). As the story develops, we see that the animals, except for the pigs and dogs, are no better off than they were under Farmer Jones.