What is the APK format?
Android Package Kit, or the APK for short, it is package file format that is used by the Android operating system for distribution and installation of mobile apps and middleware. It is a type of container file that is used to contain the files of the mobile apps in order for easier download and transfer.
An APK file contains all of that program's code (such as .dex files), resources, assets, certificates, and manifest file. The file format typically has a Filename extension ending in .apk, or sometimes in .xapk. The APK is extended from the zip format packages based on the JAR file format.