APK stands for Android Application Package, which is a file format essential for distributing, installing, and managing mobile applications on the Android platform. An APK contains all the necessary code, resources, and instructions for an app to run on Android devices. Key components of an APK include DEX files (app code), assets and resources (images, sounds, layouts), certificates and security signatures (for verification), and the AndroidManifest.xml file (which outlines permissions and components). When an app is installed, the APK is unpacked, permissions are granted, and its integrity is verified through digital signatures. APK files support various applications and games, allowing for manual installations and updates, and they ensure universal compatibility across devices. Users can adjust security settings to allow installations from unknown sources, but Android restricts this by default for safety.