An APK (Android Package Kit) is the file format used to install applications on Android devices. It contains all the elements an app needs to function code, resources, assets, and manifest files. APKs are similar to .exe files on Windows. Users can download APKs from the Google Play Store or install them manually through a process called side loading. This is useful for apps not available on the Play Store. However, users should be cautious when installing APKs from unknown sources to avoid malware or security threats.