At the beginning of the book, or at least after the revolution where the animals take over the farm, the leaders come up with seven commandments.
Over the course of the book, the commandments are changed many times, but eventually they all get boiled down to just one commandment. By the end of the book, the only commandment is this
All animals are equal, but some animals are more equal than others.
The process of boiling down the commandments shows us a couple of things. It shows us how the leaders are willing to try to change "history" to suit themselves. And it shows how the revolution has gone from something very egalitarian to something where class differences have become very important again.
After he revolution and the development of the seven commandments, the idea of creating a different society that would be better than the way mankind exists is strong and idealistic. However, it does not take long for the animals to begin breaking the commandments or re-establishing them so they fit their wants.
One of the laws was not to sleep in beds, but soon the pigs are in the beds. Another rule was not to work with mankind, but soon the animals are trading with man. Just like mankind had taken the original Ten Commandments and manipulated them so that they worked to allow some of the sins that man wanted to commit, the animals have done the same.
In the end the commandments have been adjusted so that they really are only commandments that can be used when those in charge see fit to hurt another with them instead of them serving their true purpose.
They changed into:
Four legs good, two legs BETTER.
No animal shall sleep in a bed, WITH SHEETS.
No animal shall drink alchohol, TO ACCES.
No animal shall wear clothes, IF DO NOT NESSESARY.
No animal shall kill any other animal, EXCEPT THE LEADERS.
All animals are equal, BUT SOME ARE MORE EQUAL THAN OTHERS.