.app - Mac Applications
To see what is inside an application you can open it with Finder. Select it with a single click with the mouse and choose "Show Package Contents" from the "Gear" (menu)button. Now you can browse the folders inside tha applications to see what's inside.
An application is a special kind of 2Bundle". See Anatomy of a Bundle
If you want to make an application, a simple way is to use Automator, and use the features other applications.