APK stands for Application Package Kit is an android file format. this contains all the application files bounded within a file. All Android applications have the extension .apk which is used for installation of android application or middleware on the Android powered device.  

an apk file contains all the codes, .dex files, manifest file, certificates, resources, and assets. these files are types of archives based on jar file format.

whether a user downloads an android application from an unofficial site or from a google play store, they are downloading an apk file.