There was a movie version made in 1955, but I have not seen that one recently so I will discuss the more recent made-for-tv adaptation from TNT in 1999. One of the most significant differences between the two versions is in the narration. The book has a third person narrator that is not quite omniscient. The movie is narrated by Jessie, a Border Collie.
The plot of the movie basically follows the book, although there is some creative license taken. For example, the movie shows propaganda films made by the pigs after they discover that the television keeps the animals entertained.