Old Major is based on Vladimir Lenin, a communist and Russian revolutionary who based his economic policies on the philosophy of Karl Marx. Napoleon is based upon Joseph Stalin, who came into power after Lenin. He also based his theories on Marxism. Stalin manipulates Marx's theories for his own purposes. Likewise, Napoleon takes the theories handed down to him from Old Major (Lenin) and Marx (not represented in the novel), and utilizes them to suit his own purposes.
Napoleon's economic policies therefore correspond to the policies of Stalin. And Stalin's brand of Marxism is called Stalinism. Snowball represents Leon Trotsky, one of the leaders of the Russian Revolution. Trotsky believed that the communist/socialist revolution should be spread globally. Likewise, Snowball argued that the animals should encourage rebellion on neighboring farms. However, Napoleon has Snowball ousted from the farm. Stalin and Trotsky had disagreements as well, so Trotsky was also ousted from power.
The economic policies under Napoleon are therefore closest to the economic policies of Stalin. Note that Stalin's policies are different from Marx's original philosophy. Marx claimed that the capitalist regime (Mr. Jones and the human capitalists) would eventually wither away with the growing tensions between capitalists and the working class. However, Stalin believed that they should not wait for this event, so he resorted to expanding state power and enacting communism with force.
The basic idea of Marxism is to have the working class control the means of production. In short, everyone controls everything and everything is therefore shared. The animals rebel in order to control the farm. Thereby, they can determine how much they work, what the conditions will be, how much they will be paid (food), and how they will govern their society. Napoleon/Stalin opts for too much power and ends up neglecting these primary tenets of Marxism, leaving the animals in conditions as poor those under Mr. Jones.
Hitler is represented by Frederick, a neighboring farmer who makes a deal with Napoleon but then cheats him. Hitler had a non-aggression pact with Hitler during World War II, which Hitler later broke.
Animal Farm doesn't delve too deeply into actual economic practices. The idea of the animals rebelling to control the farm corresponds to Stalin's forced communism. (This differs from Marx's idea that socialism would gradually take over for capitalism.) But it does show how the basic policies of Stalin devolved into an oppressive state. As Stalin's (Napoleon's) oppression grew, so did his policies. He used the official newspaper, Pravda, to spread this propaganda. In the novel, Napoleon uses Squealer to spread the propaganda, and this includes changes to the tenets of Animalism. These changes are all to enforce the growing power of Napoleon's regime.