What is the book about and what does the story mean?

Animal Farm is a cautionary tale about how easily people can be duped by a dictator if they are not careful, especially if they are not careful about how language is used. It is an allegory, or retelling, of how the Russian Revolution, which began with the great hope of establishing a state run for the benefit of working people, turned into the nightmare of Stalinist totalitarianism.
Orwell transforms the story of the Russian Revolution into a revolt by animals who take over their mismanaged farm and decide to run it themselves. They start out with high hopes that they can create a society based on the idea of all animals as equal. At first, they all work hard together for the common goal of survival. They decide on Seven Commandments to rule the animals. These commandments not only declare that all animals are equal, but also outline a way of life that rejects human standards. They sing a song, "Beasts of England," that uses rich imagery to envision a future state in which all animals live prosperously.
Almost from the start, however, the pigs, who are the most intelligent animals, begin to set themselves apart and take more privileges. At first, they make this sound reasonable. After Napoleon, a pig based on Stalin, takes over, however, the other animals lose more and more privileges, and their language is simplified and dumbed down. By the end of the story, all the animals but the pigs are in the same—or worse—state of poverty they were in when Farmer Jones ran the farm. The pigs are behaving just like humans—in fact, they have become human. They gradually twist the Seven Commandments into one: all animals are equal, but some are more equal than others.
Orwell wants people to be careful so that those with evil intent don't twist language to seize control of a country and oppress everyone else.