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 work, including code, resources, assets, and a manifest file. APKs are similar to .exe files on Windows. Users can download APK files from the Google Play Store or other sources and install them manually. This allows early access to updates or apps not available in certain regions. However, installing APKs from unknown sources can pose security risks. Always verify the source before installation.