Animal Farm was written by George Orwell as an allegory for the Russian Revolution.
First of all, all characters from this short novel represent a person, idea or movement. For example, Mr. Jones represented Czar Nicholas II, and he had the same characteristics as the Czar. Mr. Jones was irresponsible cruel and not fit for his duty.
Other examples are Old Major and Snowball. Old Major represented Karl Marx who was the creator of communism, and consequently in the story Old Major taught "animalism". Snowball represented Leon Tolstoy, one of the leaders of the October Revolution and a very smart and idealistic person, consequently Snowball led the revolution at the farm and had the same personal characteristics as Tolstoy.
Finally, Napoleon represented Joseph Stalin who was corrupt, selfish and only cared for power.
The Russian Revolution was a process that overthrew the Czar and established Lenin's regime. This situation is directly mirrored in the story because the same happens, there's a revolution and subsequently a new hard and unfair regime is established.