The Model is simply something that holds state that your Controllers and Views communicate with. The focus of the
pattern primarily a separation of concerns between components. In other words, it is not as helpful to say "X class is the model" so much as to say "there is no persistence code in my controllers or views".
There could be arguments made looking at the same architecture as to what classes are the Model. One could argue EJBs, another could argue Transfer Objects, still another might refer to the entire backend as the Model. But that's really not the point of the pattern.
The MVC identifies three types of code: Model, Controller, and View. Following this pattern is simply a matter of ensuring that no component contains more than one of these 3 types.