This novel is famous, and deservedly so, for its biting critique of Soviet history and rhetoric. This novel is an allegory of the rise to power of Josef Stalin. This is why we see the overthrow of the human oppressor Mr. Jones by a coaliltion of the animals on the farm. However, the pigs rapidly consolidate power for themselves after the "revolution", replacing the democratic coalition that had been forged. This of course mirrors the seizing of power by the Soviet intelligentsia, who positioned themselves as the new ruling class in Soviet Russia.
The in-fighting and struggle for dominance between Trotsky and Stalin is mirrored by the feud between the two pigs, Snowball and Napoleon. In both cases, the idealistic but less powerful person (Snowball and Trotsky) is expelled by the more rapacious and powerful figure (Napoleon and Stalin). We see that the purges and show trials feature in this novella too, with the false confessions and execution of animals that Napoleon distrusts after the windmill. Stalin's abandonment of the original principals of the Russian Revolution and his despotic rule find their parallel in the establishment of the tyrannical government of the pigs and their taking on the trappings of their former oppressors.
This raises one of the key messages of this novella. Orwell is not writing about tyrannies alone, he is rather writing a harsh condemnation of those revolutionary movements that begin with principals such as freedom and equality, but then quickly use this ideology to secure power and perpetrate even greater injustices than existed before.