An APK (Android Package Kit) is the file format used by Android to distribute and install apps. It contains all the elements an app needs to function properly on a device, including code, resources, assets, certificates, and manifest file. APK files are similar to .exe files on Windows. Users can download APKs from the Google Play Store or third-party sources. Installing from unknown sources requires enabling settings on the device. APKs allow manual app installation, useful for beta versions, updates, or apps not available in certain regions. Always ensure APKs are from trusted sources.