APK is a format specifically for Android devices. It uses a .apk extension and can be downloaded via Android software sources.
 The format
This file format is a variant of the JAR format and is used for the distribution and installation of bundled components onto the Android mobile device platform.
An APK file is a ZIP file, with a particular internal file layout that allows it to be run in place, without unpacking. This format usually consists of 2 folders and several files. This includes:
- META-INF (folder)
- res (folder)
- classes.dex - a compiled Dalvik Executable.