An APK (Android Package Kit) is the file format for applications used on the Android operating system. APK files are compiled with Android Studio, which is the official integrated development environment (IDE) for building Android software.
An APK file includes all of the software program’s code and assets. Developers who create applications for use on Android devices must compile their application into the APK format prior to uploading to Google Play, the official marketplace for Android applications. Google Play requires APK files be less than 100 MB in size. To help developers with this limitation, Google Play will host up to two APK expansion files for any additional content required for their application. An APK expansion file is used for storing media files, high-fidelity graphics, or other large assets that would cause an APK to exceed the size limitation.
APK files can also be distributed directly to other Android users for installation on their devices. Android users can grant permission to their device to install unknown apps if they wish to access APK files from another source and install them directly. Android users may wish to install an APK directly if they are beta testing an unreleased version of an application, or due to a device restriction, are unable to download an app from Google Play.