Both Napoleon and Snowball are pigs living on Manor Farm. Both become leaders in the revolution, although their roles and responsibilities take them in very different directions.
Napoleon is a "large, rather fierce-looking Berkshire boar" who becomes the dictator and source of ultimate power in the government set up after the revolution. Napoleon exercises his authority behind the scenes; he seldom comes out among the other animals, and always has his guard dogs around to protect him through intimidation or attack, as needed. The other animals fear Napoleon but also credit him with the success of Animal Farm after the humans are expelled.
It had become usual to give Napoleon the credit for every successful achievement and every stroke of good fortune. You would often hear one hen remark to another, 'Under the guidance of our Leader, Comrade Napoleon, I have laid five eggs in six days'; or two cows, enjoying a drink at the pool, would exclaim, 'Thanks to the leadership of Comrade Napoleon, how excellent this water tastes!
Snowball is the spokespig and the organizer of the revolution. He is "quicker in speech and more inventive" than the other pigs and becomes the conveyer of messages and explanations between the pigs and the other animals. When questions begin to arise about the ways in which things seem to not be working out as the other animals anticipated, Snowball is the one to prove that all is working out exactly as it should have been.
Reading out the figures in a shrill, rapid voice, he proved to them in detail that they had more oats, more hay, more turnips than they had had in Jones's day, that they worked shorter hours, that their drinking water was of better quality, that they lived longer, that a larger proportion of their young ones survived infancy, and that they had more straw in their stalls and suffered less from fleas.
Snowball is eventually driven off the farm and is blamed for everything that goes wrong. He is made into a villain in the eyes of the animals, while Napoleon goes on to establish renewed relationships with the humans of surrounding farms.