APK (Android Package) is the file format used for distributing and installing applications on Android devices. It contains all the necessary components, including code, resources, assets, and manifest files. APKs are typically downloaded from the Google Play Store, but they can also be installed manually from third-party sources. However, installing APKs from untrusted sources may pose security risks, such as malware infections. Developers use APKs to test apps before publishing. Users can extract APK files from installed apps using special tools. To install an APK, "Unknown Sources" must be enabled in settings.