Model = code for CRUD database operations.
Controller = responds to user actions, and passes the user requests for data retrieval or delete/update to the model, subject to the business rules specific to an organization. These business rules could be implemented in helper classes, or if they are not too complex, just directly in the controller actions. The controller finally asks the view to update itself so as to give feedback to the user in the form of a new display, or a message like 'updated, thanks', etc.,
View = UI that is generated based on a query on the model.
There are no hard and fast rules regarding where business rules should go. In some designs they go into model, whereas in others they are included with the controller. But I think it is better to keep them with the controller. Let the model worry only about database connectivity.