what is a hook?

In George Orwell's classic novella Animal Farm, Old Major's utopian vision of a farm for all animals degenerated into an autocratic dictatorship. The destruction of Old Major's vision can be seen through key elements of the storyline. The Pigs taking over, Boxer the horse's tragic life and Napoleons autocratic reign. I will give examples of how the pigs took over and how that changed what Old Major invisioned for the farm.

....which is the hook in thiss paragrgh?