The book tells the story of the Russian Revolution and the subsequent rise of Stalin, only the events take place on a farm. The deposed czar is a drunk, incompetent farmer who mistreats the animals, who all represent different figures or types of people in society. The pigs, cleverest of the creatures, are the leaders of the revolution, and two emerge on top afterwards - Napoleon (Stalin) and Snowball (Trotsky).
Just the fact that it's animals makes some of these events so amusing, as when Napoleon gets one of his propaganda pigs to write a song in his honor. Here's one verse:
Friend of the fatherless!
Fountain of happiness!
Lord of the swill-bucket! Oh, how my soul is on
Fire when I gaze at thy
Calm and commanding eye,
Like the sun in the sky,
But then, the amusement always fades after each of these incidences when it hits you, yet again, that this actually happened to people. That there are still places like this. And if you view what goes on from the outside, it's a combination of the deeply horrible and the undeniably ridiculous. That's a part of the genius of Animal Farm, showing this. Because it's ridiculous when a bunch of sheep chant, "Four legs good! Two legs bad!" But it's also depressing: sad enough when actual sheep do it, and much worse when you think about the real-life people they represent.
I forget which edition I read, but there was an intro that discussed how thankfully the dystopian visions of writers such as Orwell and Huxley don't seem to have panned out, and that the books were important warnings but that the human spirit has triumphed (or something of the sort)... and yet, they're still relevant and always will be. When you read Animal Farm, you're reminded of the kinds of things you see in politics all the time: the brazen rewriting of the past (counting on people's short attention span, ignorance, or intimidation to get away with it), the chanting of inane slogans to drown out meaningful debate, the power-grabs and erosion of rights, the push for a utopia that will just cause more misery, new idealistic politicians turning into the same old corrupt ones and rationalizing their crimes as actions taken for the greater good...
Anyway, if you haven't read it, do so. It's a great book. And you could finish it in an afternoon; it just breezes by.