One major difference is the fact that Fern takes Wilbur to school and is taken to the principal’s office. This event does not occur in the novel.

Another major difference between the two representations is the way in which Wilbur learns about the slaughter of pigs and the farmer’s plan for his life. In the movie, Wilbur discovers the smoke house after he has escaped from his pen. At this point, the animals explain what will happen to him in the future. In the book, Wilbur is unaware of what is planned for his future until the animals inform him.

One other difference is the animal’s reaction to Charlotte when she is first introduced. The animals are portrayed as being afraid of her and somewhat reluctant to be around her in the film version. When she is introduced in the book, however, the animals do not have a strong reaction to her.

http://beck518.blogspot.in/2009/08/book-vs-movie-comparison-charlottes-web.html