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 app to function on a device, including code, resources, and manifest files. APK files are similar to .exe files on Windows. Users can download APKs from the Google Play Store or install them manually from other sources. However, installing APKs from unknown sources can pose security risks. Developers often use APKs to test apps before official release or share apps outside the Play Store.