This document discusses the Model-View-Controller (MVC) pattern in the context of a DJ music application called DJView. It goes through multiple versions of the DJView code, refactoring it to better separate the model, view, and controller responsibilities according to MVC. The model manages the application data and logic. The view displays data and handles user input. The controller mediates between the model and view, updating the model based on user input and notifying the view of model changes.