At first, the major difference between the pigs and the other animals is that the pigs are more intelligent; it is never specified whether they have superior brains or are just willing to study harder, but they learn to read and write long before the others. As the farm begins its run under animal ownership, the pigs set themselves apart from the others by assuming supervisory roles: they figure things out but do little actual labor.
The pigs did not actually work, but directed and supervised the others. With their superior knowledge it was natural that they should assume the leadership. Boxer and Clover would harness themselves to the cutter or the horse-rake... and tramp steadily round and round the field with a pig walking behind and calling out [directions].
(Orwell, Animal Farm, msxnet.org)
Later, the pigs begin to give themselves luxuries that the other animals do not get; they move into the farmhouse, sleep in beds, consume more and better food, and drink alcohol. In this way, the pigs distance themselves as "leaders" and "brainworkers" from the "working class" of all the other animals. In time, the animals are conditioned to accept that the pigs receive better treatment, better food, and have ultimate authority despite performing no real labor in support of the farm and the common good.