OutSystems apps consist of modules that you bootstrap from an application template. These make development efficient because the apps based on application templates have many predefined elements. For example, login screens, a theme, layout blocks.
There are built-in, custom, and Forge application templates.
Built-in application templates
Reactive Web App, Phone App, Tablet App and Traditional Web are the built-in application templates created and maintained by OutSystems. They reference the OutSystems UI Framework and enable you to use its theme, patterns, screen templates and other components.
Custom application templates
These are the application templates you create, based on the UI framework of the source application templates. The document Create a Custom Application Template explains how to create these templates.
Forge application templates
These are application templates provided by the OutSystems Community and shared at Forge.