An APK (Android Package Kit) is the file format used to distribute and install applications on Android devices. It contains all the necessary components of an app, including the code, resources, assets, certificates, and manifest file. APK files allow users to install apps manually outside the Google Play Store, often used for beta testing or apps not available in certain regions. However, downloading APKs from untrusted sources can pose security risks, so it's important to verify the source before installation.