Android application package (APK) is the package file format used by the Android operating system  installation of mobile apps andmiddleware.

APK files can be installed on Android powered devices just like installing software on PC. When a user downloads and installs an Android application from either an official source or from some other  site, they are installing an APK file on their device.

A user or developer can also install an APK file directly to a device  from a desktop computer.

An APK file is an archive that usually contains  following files and directories:

- single .dex file

-zipped resources

-non java libraries