An APK (Android Package Kit) is the file format used by Android to distribute and install apps. It contains all elements an app needs to install correctly, including code, resources, assets, and certificates. APK files are similar to .exe files on Windows. Users can download them from the Google Play Store or install them manually via side loading. Developers use APKs to test and share apps. It's important to only install APKs from trusted sources to avoid security risks.