An APK (Android Package Kit) is the file format used by the Android operating system to distribute and install applications. It contains all the necessary components, such as code, resources, assets, and manifest files, required for an app to run on an Android device. APK files are similar to EXE files on Windows or DMG files on macOS. While most users download apps from the Google Play Store, APK files allow for manual installation, enabling access to apps not available in official stores. However, installing APKs from untrusted sources can pose security risks.