Orwell's main message in Animal Farm is that power corrupts, even when idealism is at play. The events of the story are an allegory for the Russian Revolution of 1917, where the bolsheviks overthrew the tsar in order to establish a communist regime. While the ideals of this new order were meant to foster equality for the oppressed masses, instead the Soviet Union became a totalitarian state in which propaganda was commonplace, so-called enemies of the state were punished brutally, and only a privileged few were allowed comfort and power.

Orwell's story shows in detail how ideals can become corrupted by power. At first, the animals believe they are all living equally, but when the pigs become more power-hungry and create excuses for giving themselves special privileges, a new hierarchy is created, in many ways replicating the old one (note how the pigs come to resemble the humans they overthrew). The novel allows Orwell to point out the dangers of such radical forms of ideology, the way a brainwashed populace can be willing to excuse the most horrific behavior from the state, and the way in which tyrants hold onto their power through propaganda and crafty rhetoric. Though he was specifically reacting to the Soviet Union, Orwell's warning can be applied to any totalitarian state.

In Animal Farm, George Orwell attempts to lay bare the hypocrisy, brutality, and moral corruption at the heart of the Soviet Union under Stalin.

At the time when Orwell wrote the book, a disturbingly high proportion of leftist intellectuals in Western Europe and the United States genuinely believed that the USSR was some kind of socialist utopia which provided an example for the nations of the capitalist West to emulate. Orwell aimed to challenge this distorted worldview by exposing the realities of life under one of history's most notorious mass-murderers.

As has been stated in a previous answer, Animal Farm is a political allegory. The farm depicted in the story is meant to represent the Soviet Union under Stalin, just as Stalin himself is represented by Napoleon, the dictator pig.

All of the events that happen in the story are based on things that actually occurred in the USSR in the 1930s, whether it's the brutal murder of the regime's enemies—both real and imagined—the constant regurgitation of mindless propaganda, or the use of famine as an instrument of repression.

Orwell, as a man of the Left, didn't want to foreclose the possibility of radical political and economic change in society. However, in Animal Farm, as elsewhere in his writings, he highlights the potential dangers implicit in any form of radical ideology when it is applied to real-world conditions.

Orwell's classic novella Animal Farm is a cautionary tale about the dangers of consolidating political power in a communist state and a warning against authoritarian regimes and dictators. Orwell's novella allegorically represents the events that transpired before, during, and after the Bolshevik Revolution in Russia, when Joseph Stalin usurped power and oppressed the entire Soviet Union. Similarly, Napoleon usurps power in the novella shortly after the Battle of the Cowshed and develops into a tyrannical ruler, who oppresses his subjects by forcing them to work long hours while continually reducing their food rations. Orwell's message warns readers about allowing shrewd, selfish politicians to consolidate power and gradually take away civil rights and liberties. He also warns readers about the various methods of manipulation and propaganda used by authoritarian regimes to oppress and control the populace. Squealer acts as Napoleon's mouthpiece and cleverly manipulates the animals using various rhetorical devices and propaganda strategies to deceive the animals into believing and supporting Napoleon's selfish, oppressive political agenda. Orwell's novella also emphasizes the dangers of government corruption and illustrates how maniacal leaders and debased politicians can establish and create an oppressive, terrifying society, where the majority of the population lives in fear.

Since Animal Farm is an allegory based on the problems resulting from Russian Revolution, and its subsequent oppressive communist state, one of the messages of the novel is about how power can often lead to corruption and oppression. Even when a revolution is done with the best intentions, all directed toward the greater social good, it can devolve into a government which is just as bad (or worse) than the oppressive regime which it replaced. 

Other messages or lessons of the novel include the means by which a government rules and/or oppresses its people (or animals in this allegory). When the revolution begins, it is to establish the law of the land that all animals are equal. However, over time the pigs (leaders) clearly put themselves in a higher position (under Napoleon) and this reestablishes a hierarchy (which was what the revolution was supposed to have eliminated). This hierarchy divides the animals; thus, they are no longer all equal. 

The novel also shows the ways a group/government can manipulate and brainwash its citizens through the use of propaganda. Napoleon and Squealer constantly change the seven commandments in order to suit their increasing power. By the end of the novel, the commandments read less like a document stating the equality and happiness of all animals, and it reads more like the establishment of the privileges of pigs over all animals. Napoleon and Squealer not only change the commandments (usually acting as if they had never been changed); they also change history to suit their narrative. Snowball had been the hero of the Battle of the Cowshed, but in order to praise Napoleon and criticize Snowball, Squealer gradually changes the story, eventually making Napoleon the hero of the battle. At the end of Chapter 5, Squealer says:

And as to the Battle of the Cowshed, I believe the time will come when we shall find that Snowball's part in it was much exaggerated. 

Using propaganda and fear, the novel shows how even a revolution with intentions of total equality can devolve into an oppressive state. The further message is about the tendency for power to corrupt. As Napoleon's regime gained power and privilege, the corruption increased as well. 

George Orwell’s novel Animal Farm warns readers about the many dangers of totalitarianism. Specifically, the book is an allegory for the Russian Revolution. Orwell was a fervent critic of Stalinism and wrote this book to show how the increased consolidation of power can become an oppressive force.

Consider how the animals dream of equality and peace and how then the pigs ultimately destroy these dreams. In the end, Napoleon’s leadership comes to mirror that of the humans whom the animals revolted against. Those who suffered before the rebellion continue to suffer. This ending shows how the utopian theories fueling communism, even though they seem great at first, actually result in totalitarianism.

But this book goes beyond warning readers about political ideologies. On a deeper level, the story warns readers about human behavior in general and how barbaric humans can be. Recall how there is no specific time that the story is set in and how it happens on an imaginary farm. By keeping the setting so general, Orwell is showing how at the end of the day, people in any time and place can be no better than animals. Just like in the book, there will always be people who are “pigs” who exploit their privilege and oppress others.

Animal Farm is an allegory about what went wrong in the USSR, which initially adopted socialism as an idealistic, utopic experiment meant to liberate workers from exploitation and share the fruits of labor equally, but which quickly turned into a totalitarian regime that robbed people of their rights and their labor. Orwell, writing at a time when many people still defended the Soviet Union, meant to show that a well-intentioned revolution can easily derail. Key to this is language: the pigs are easily able to control the rest of the animals because they control the rhetoric. Language is very important, Orwell argues. We have to maintain vigilance over what is said and how it is said. The animals end up deceived because the pigs continually change the rules of the game through word games, finally changing "all animals are created equal" by adding to it the famous non-sequitur that some are more equal than others.

Violence--or putting controls on it-- is also important to maintaining freedom. Orwell illustrates this by showing how the pigs maintain a monopoly of violence by controlling the dogs, who act as executioners against the other animals. This combination of rhetoric and violence keeps the rest of the animals down, which is, of course, what happened under Soviet socialism--and more to the point, could also happen in nominally democratic states like Orwell's home county, England--or in the United States.