What is MVC and how it maps to Struts?

MVC is a design pattern and struts is  a framework used to implement MVC

Model-View-Controller (MVC) is a design pattern put together to help control change. MVC decouples interface from business logic and data.

Model : The model contains the core of the application's functionality. The model encapsulates the state of the application. Sometimes the only functionality it contains is state. It knows nothing about the view or controller.
Action class in struts.

View: The view provides the presentation of the model. It is the look of the application. The view can access the model getters, but it has no knowledge of the setters. In addition, it knows nothing about the controller. The view should be notified when changes to the model occur.
JSP in struts

Controller:The controller reacts to the user input. It creates and sets the model.
ActionServlet and RequestProcessor class in struts.