An application in the Mac. Although files with an APP extension appear to be single files, they are actually a hierarchy of files and folders that contain the executable program, other software components and data. Known as an application "package" or "bundle," the APP file extension is used with several other operating systems. iOS Mobile Apps Apple's iOS apps also use the APP package, although for ARM hardware rather than Intel. When submitting to the App Store, the Xcode development system compresses (zips) the APP package along with iTunes images and meta-data into an archive with iPhone App (IPA) extension.