What is the significance that at the end, it is impossible to tell the pigs apart from the people?
The final scene of Animal Farm, where the animals watch with confusion as the pigs play cards with the humans, shows that the principles of the revolution, and of the society it created, have been completely corrupted. The pigs have become what they led the fight to overthrow:
No question, now, what had happened to the faces of the pigs. The creatures outside looked from pig to man, and from man to pig, and from pig to man again; but already it was impossible to say which was which.
This change did not happen overnight, nor was it ever evident to most of the animals that it was happening. From the very beginning of the revolution, when the pigs appropriated the apples and milk for themselves, to the end, when the Seventh Commandment was altered to "Some Animals are More Equal than Others" and Squealer taught the sheep to chant "Four legs good, two legs better," the principles of Animal Farm had been subverted by its own leaders. By the end of the book, we learn that "Animal Farm" has been renamed "Manor Farm," that the pigs walk around in human clothing, and that the life of the animals is no better than it was before the revolution:
As for the others, their life, so far as they knew, was as it had always been. They were generally hungry, they slept on straw, they drank from the pool, they laboured in the fields; in winter they were troubled by the cold, and in summer by the flies. Sometimes the older ones among them racked their dim memories and tried to determine whether...things had been better or worse than now. They could not remember.
A revolution aimed at improving the lives of the animals, with the best possible intentions, has degenerated into a totalitarian nightmare. The parallels with Stalin's Soviet Union were not lost on Orwell's readers.