The farm animals who inhabit the Manor Farm are mistreated and abused by Farmer Jones. The animals rebel, expel Jones, and take over the farm, which they rename Animal Farm. Soon, however, the pigs (who represent the party bosses) begin to take special privileges for themselves, e.g. extra food. They enlist the farm's dogs as enforcers to put down any dissent, and they teach the sheep (rank and file) to speak the party line on demand. At first, this is, "Four legs good, two legs bad"- animals (four legs) are good, humans (exploiters) are bad.
As time passes, the Seven Commandments (Animal Farm's Constitution) undergoes subtle changes as the pigs rewrite it to suit their own agenda. When Boxer the horse (symbolizing blue-collar labor, the "workers,") becomes too old to work, the pigs sell him to the horse butcher, whom they tell the other animals is really the veterinarian. The pigs eventually learn to walk on two legs, thus imitating the animals' original exploiters, and they teach the sheep to bleat, "Four legs good, two legs better!" The Seven Commandments become one: "All animals are equal, but some animals are more equal than others." The pigs rename Animal Farm the Manor Farm- its original name- and invite the neighboring human farmers, who symbolize the elite class against whom the animals revolted, to admire the results: "...the lower animals on Animal Farm did more work and received less food than any animals in the county." As the story ends, the pigs become indistinguishable from their human visitors.
(excerpt taken from http://www.stentorian.com/animfarm.html#synopsis)