Model 1 embeds navigation in the JSPs. It tends to be brittle when the number of pages gets large.
Model 2 combines JSPs with a servlet, usually a single front controller, which handles the navigation chores and leaves the JSPs to concentrate on presentation. All requests are sent from JSPs to the front controller servlet. Some purists argue that because of missing out on the push aspect (the view pulls the data from the model in order to display tables, etc) of the true Model-View-Controller pattern, the Model 2 architecture shouldn't be called MVC. Nevertheless, the term "web MVC" is here to stay.
Struts and Java Server Faces are based on model 2 MVC.