An APK (Android Package Kit) is the file format used by Android to distribute and install apps. It contains all the elements needed for an Android app to function, including code, resources, assets, and manifest files. APKs are similar to .exe files on Windows. Users can download APK files from the Google Play Store or third-party sources to install apps manually. Installing APKs from unknown sources requires enabling permission in device settings. APKs are useful for testing apps, accessing apps not available in certain regions, or getting early access to updates. However, they should be downloaded from trusted sources to avoid security risks.