Satire is when an author uses "humor, irony, exaggeration or ridicule" to "expose and criticize" an individual or society as being either foolish or morally corrupt ("Satire").
In Chapter IV of Animal Farm, George Orwell satirizes Russia's Bolshevik Revolution, in which the Bolshevik working class rose up against Russia's aristocracy, by characterizing the working class as farm animals and the aristocracy as farm owners. The Bolshevik Revolution began on October 2th, 1917, so it is also called the Great October Socialist Revolution, Red October, or the October Uprising ("October Revolution").
In particular, Orwell exaggerates the corruption of the aristocracy by describing Mr. Jones and the other farmers as wallowing in states of drunkenness in the "taproom of the Red Lion at Willingdon."
Orwell also uses humor to satirize the real-life battle that took place in October. Specifically his descriptions of the attacking animals can be seen as humorous, as in the following:
All the pigeons, to the number of thirty-five, flew to and fro over the men's heads and muted upon them from mid-air.
What's more, since it is illogical to think farm animals can win a battle against humans, we can also see his portrayal of the Bolshevik Revolution as an exaggeration and humorous.
Satire is defined by dictionary.com as "the use of irony, sarcasm, or ridicule to expose, denounce, or deride people's vices." Often satire is used against political structures and states too.
Chapter four of George Orwell's Animal Farm makes use of irony. Mr. Jones plans to attack the animal farm with the help of some of his neighbors. The animals are alerted to the attack by a flight of pigeons and begin preparing a defense. The animals are hugely successful and suffer only a single casualty. Medals are given, burial ceremonies occur, a holiday is established, and the animals are once again under the illusion that their way is the best way.
The irony is that the animals believe that they are defending themselves against an oppression, namely a human oppression. What most of the animals do not realize is that they are further cementing the pigs' power which will be just as oppressive (if not more).