.app is the extention of application packages in Mac OS X (and its NeXTStep predecesors). .app packages are really just that, packages! They are each their own little directory and contain resource files as while as compiled executables. The usefulness of these resource files is clear when the progam interface file can be modified, changing the application's menu layout or look without needing the source code. Doing this is very similar to using ResEdit on Mac Classic systems.