An APK (Android Package Kit) is a file format used to install apps on Android devices. It includes all necessary components like code, resources, and certificates. APKs allow users to install apps manually, especially from third-party sources. However, downloading from untrusted sites can pose security risks. Developers use APKs for testing before official release. Some modified APKs offer extra features but may violate policies. To stay safe, always download APK files from verified sources like the Google Play Store.