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, including code, resources, and metadata. APK files are similar to .exe files in Windows. Users can install APKs from the Google Play Store or manually through “sideloading.” This is useful for accessing apps not available in certain regions or testing new versions. However, downloading APKs from untrusted sources can pose security risks, so it's important to verify the source before installation.