An APK (Android Package Kit) is the file format used by Android to distribute and install apps. It contains all necessary components like code, resources, and metadata. Users can install APKs manually from third-party sources outside the Google Play Store. However, downloading from unknown sources can pose security risks. APKs are useful for early access to apps, region-locked applications, and app updates before official releases. Always verify APK sources to avoid malware and ensure device safety.