TApplication, TScreen, and TForm are the classes that form the backbone of all applications by controlling the behavior of your project. The TApplication class forms the foundation of an application by providing properties and methods that encapsulate the behavior of a standard program. TScreen is used at runtime to keep track of forms and data modules that have been loaded as well as maintaining system-specific information such as screen resolution and available display fonts. Instances of the TForm class are the building blocks of your application's user interface. The windows and dialog boxes in your application are based on TForm.