An APK (Android Package Kit) is the file format used by Android to distribute and install apps. It contains all the necessary components an app needs, including the code, resources, assets, certificates, and manifest file. APK files make it easy to share apps outside the Google Play Store, allowing manual installation sidloading on Android devices. Users must enable "Unknown Sources" in settings to install APKs manually. Developers often use APKs to test apps before official release. However, downloading APKs from Untrusted sources can pose security risks like malware, so it's important to only use verified sources.