The pigs, particularly Napoleon, abuse their power. Napoleon represents Josef Stalin, the "communist" leader who ruled Soviet Russia as a totalitarian dictator. The other pigs represent the small power elite that coalesced around Stalin.
Like Stalin and his cohort, the pigs betray the ideals of their revolution. The initial idea of the animal revolt, expressed by Old Major, the boar standing for Karl Marx, was that, having gotten rid of the human owners, all the animals would be equal. Having seized the farm, they would use its profits to make life materially better for all the animals. Further, the animals would not behave like the humans. They would not live in houses, wear clothes, sleep in beds, or drink alcohol.
Instead of being true to these ideals, the pigs use trained dogs to terrorize the other animals. The pig stage trials and executions, gradually take away all the rights of the other animals, and commandeer all the farm's profits for themselves. The pigs live in a house, wear clothes, sleep in beds, and act in all ways like humans. The other animals end up worse than they were before.