APK stands for Android package kit. This is a file format that is used by the operating system in Android for the installation and the distribution of middleware and mobile apps. The files are actually analogous to the other packages like APPX for Microsoft Windows or the Debn kind of packages within debian based kind of OS such as Ubuntu.
So as to come up with the APK file, one needs an Android program for Android to be compiled and then all parts packaged into a single file that contains the code of the program. This can be files like .dex files. The APK file also has to have the manifest, certificates, assets and resources files.