Main characters:
Napoleon, a pig, a 'smart' pig. He takes the leadership amongst with the other pigs. He is very mean. He only does things when he gets better by it.
Boxer, a hard working cart-horse. When things are going bad on Animal Farm, he only works harder. He doesn't see that Napoleon is terrorizing the whole farm.
Squealer, a fat pig, helper of Napoleon. He does everything that Napoleon tells him to do. He is also very mean against the other animals.
Summary:
The book describes the overtaking of the Manor Farm that was owned by Mr. Jones and what happens after the revolution.
The animals living on the Manor Farm did not longer accept the oppressing by Mr. Jones. On an evening all the animals had come together to discuss how they could overtake the farm. Together they made a plan, that was implemented in due time. The Manor Farm was taken over and renamed in Animal Farm. Mr. Jones was been chased away to another farm, somewhere else in England.
After the farm had been taken over, and the common enemy was gone, the real problems came up. In the beginning, on Animal Farm all animals lived in happiness with each other. Indeed they where all running the farm together. The animals tried to learn reading but only the smartest animals actually learned it. Many animals didn't came farther than A, B, C. The pigs, who could read and write the best, proposed seven rules which all animals should obey to:
Whatever goes upon two legs is an enemy.
Whatever goes upon four legs, or has wings, is a friend.
No animal shall wear clothes.
No animal shall sleep in a bed.
No animal shall drink alcohol.
No animal shall kill any other animal.
All animals are equal.
These rules were summarised by the statement 'Four legs good, two legs bad'.
However, gradually things changed. The pigs and the dogs created a new rank. They made themselves the leaders of Animal Farm. Sometimes their behaviour had to be defended by adapting several rules of the above seven commandments. For instance, the last rule changed from All animals are equal' to All animals are equal, but some animals are more equal than others'. This process was done very subtlely and most of the animals observed the changes too late. Their criticism is consequently responded by the remark: but it was always so, don't you remember?
There is also a struggle for leadership between the pigs Napoleon and Snowball. Snowball could make brilliant speeches, and had good ideas. But Napoleon had most support from the sheep. After presenting a plan for building up a windmill Snowball was chased away by a group of dogs. After this moment Napoleon is the ultimate leader. Many ideas, originally made by Snowball, are now proposed again by Napoleon. And when things sometimes go wrong it was always Snowball who was blamed for it.
Napoleon behaves more and more like a human being. He sleeps in a bed with sheets, he wears clothes and finally he even succeeded in walking on two legs. Indeed, the statement 'four legs good, two legs bad' was changed into 'four legs good, two legs better'.
Life at the farm gets even harder then before the rebellion. There is little spare time, food is rationed, and there is a sixty-hour working-week. The horse Boxer is the best worker of all. He works harder and harder each time things go bad. In the end this nice supporter of the revolution collapses. In the mean time the pigs and dogs lead a comfortable life.

My opinion of the book:
The book is written very clearly and it has a good story. It has something to tell: it describes the history of a revolution that starts good with genuine purposes. But in the end nothing has actually been changed, a new leader for the farm has stand up, and is controlling the farm, just as it has been done before by Mr. Jones. In fact the new leader imitates the former Mr. Jones in many ways and thus is also acceptable to the farmers of the neighbouring farms.
The book is a humoristic satire on several revolutions that happened in the past: the French revolution (that started with ideas like equality), and the Russian revolution (leadership by the working class). In both cases things went wrong in the end. A new upper class aroses, that behaved in a similar way as the former one. So nothing had changed in the end. Also the struggle between the new leaders themselves is met very often. And the fact that the revolution has not been exported to other farms made the pigs acceptable to the neighbouring farmers, especially when they behaved so human like.