An APK, or Android Package Kit, is a file format used by the Android operating system for the distribution and installation of mobile applications. It contains all the necessary components, such as code, resources, and manifest files, that allow an app to run on an Android device. Users can install APKs directly, bypassing the Google Play Store, which is useful for accessing apps not available in the store. APK files are similar to other software package formats, like EXE files for Windows or DMG files for macOS. Developers create APKs to package their applications for easy installation and sharing. However, users should be cautious when downloading APKs from unknown sources, as they may pose security risks.