Wow! That's a question that requires a long answer. Basically the whole book is one huge allegory about the perils of Stalinism in particular and totalitarian governments in general.
Animals represent the different sections of society and the leaders who try to control them. For instance, the pigs are the Communists, the horses are the workers, the dogs are the army or police, and the birds are non-labor types including clergy and farmers. Sheep represent the masses, all too keen to regurgitate the motto of Animalism (Communism): "Four legs good, two legs bad."
The windmill, which was supposed to make the animals' life easier, is a reference to Stalin's Five Year Plan to improve the lot of every Soviet citizen and make Russia more competitive on the world stage. But neither the windmill nor the plan achieved their aims. When the windmill is destroyed it mimics the failure of Stalin's plan to come to fruition.
When the animals revolt, this of course represents the Russian Revolution.
What a stroke of genius to have Stalin represented by a pig (Napoleon) - while the spirit of the Russian media who spewed out Stalin's propoganda resides in Squealer.
As mentioned above, the answer could go on for pages and pages. But at least it gives a starting point in case anyone else wants to chip in.
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