FIles with APK file extension are used by Android operating system. Most Android user use APK file daily without even realizing it. APK is an app distribution and installation, and update package format, used by default by Google Play store.
Some APK packages are distributed outside of the Google Store via unofficial channels. However, Android by default prevents installation of such packages. Installing packages for unknown sources potentially exposes user’s system to various security threats as they circumvent any security precautions implemented by Google. Therefore it is very important to verify the source and author of any packages obtained outside Google Store.
Applications distributed as an APK file packages have various limitations, such as device and Android versions compatibility requirements. A typical APK distribution usually contains such information readily accessible to potential users. APK apps may also by installed using various Android emulators.