This is an extremely broad question; you might want to check out the link below. Orwell wrote Animal Farm as a political satire of the Bolshevik Revolution in Russia. Each character in the work represents a person, country, theory, etc., from the Revolution. For example, Mr. Jones, the farmer, is the czar (chased and executed by the Bolsheviks during the Revolution). Old Major represents Lenin, the "beloved" leader of the Revolution, and Napoleon symbolizes Josef Stalin, the dictator who controlled Russia after Lenin's demise.
This is simply the beginning of the symbolism. The work is an allegory, in which everything or everyone represents something else.
If you are writing a paper on this topic, you will need to narrow your question.