An APK (Android Package Kit) is the file format used to distribute and install applications on Android devices. It contains all the elements an app needs to function properly, including code, resources, assets, certificates, and a manifest file. Users can download APK files from the Google Play Store or third-party websites. Installing an APK manually allows access to apps not available in certain regions or versions not yet released. However, it’s important to download APKs from trusted sources to avoid security risks. APKs are essential for Android app development, testing, and distribution outside of official app stores.