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