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, including code, resources, and manifest files, required for an app to run on an Android device. APK files are similar to .exe files on Windows and allow users to install apps manually, bypassing the Google Play Store. They can be downloaded from third-party sources, but users should be cautious to avoid malware. Developers use APKs to test applications before official release. To install an APK, users must enable “Unknown Sources” in device settings.