How is Animal Farm book allegorical?

It is about a group of animals who overthrow the farmer and take over "Manor Farm." The story tells about these animals as they establish life on their own and as free animals. As the story progresses, the pigs, especially Napoleon, take power and dominance over all the animals. "All animals are equal, but some animals are more equal than others." It's actually a really good book. Check out spark notes for a more detailed summary.