This information is valid for Horde 5 and later only. See Doc/Dev/SpecialFilesH3 for Horde 3 or Doc/Dev/SpecialFilesH4 for Horde 4.
horde/lib/core.php: This file
There are several files with a special meaning that can be found in every Horde application:
index.php: This script is loaded if a user enters an application. Usually this script doesn't have any functionality but simple loads the default view of the application, sometimes depending on the user's preferences.lib/Api.php: This files defines and implements all methods that an application is exposing through the Registry to the other Horde applications and the Horde RPC servers.lib/Application.php: This file defines information need to initialize the application and to interact with the Horde framework. It also defines the version number of the application.lib/Exception.php: This file defines the application-specific exception class.lib/Test.php: This file defines any application specific tests to run when using the Horde test script.lib/Ajax/Application.php: This file defines the AJAX actions provided by this module.themes/default/screen.css: This is a CSS file that defines all application specific style sheet rules.