File with an .APP extension contains a pack of data of an application, executable within Mac OS X, including executable files, application’s resources, PLIST files (properties lists, plugins and icons). It can be initialised by double-clicking . It corresponds to the EXE files in Windows system.

APP files are packs, within Mac OS X their content can be viewed with an option in the context menu of a file. In other systems such files can be displayed as catalogues with their content or they may need to be extracted with an archiving program. In order to maintain proper functioning of an APP program, the content of the pack should not be modified manually.