APK (Android Package Kit) is the file format used by Android to distribute and install applications. An APK file contains all the necessary components—code, resources, assets, certificates, and manifest file—needed for an app to run on an Android device. APKs are similar to .exe files on Windows. They can be downloaded from the Google Play Store or third-party sources. Users can install APK files manually by enabling "Unknown Sources" in settings. Developers often use APKs to test apps before publishing. APK files are essential for Android app development, distribution, and installation.