An APK (Android Package Kit) is the file format used for installing apps on Android devices. It contains all the elements needed for an app to run, including code, resources, and metadata. APKs allow users to install apps manually, bypassing the Google Play Store. They are useful for early access, region-restricted apps, or older versions. However, downloading APKs from unknown sources can pose security risks, so it's essential to use trusted platforms. Developers often use APKs for testing apps before publishing them officially.