Android phones, like the Galaxy Nexus, allow installation of applications directly via APK files or indirectly via Google Play
Androidapplication package file (APK) is the file format used to distribute and install application software and middleware onto Google's Android operating system; very similar to an MSI package in Windows or a Deb package in Debian-based operating systems like Ubuntu. To make an APK file, a program for Android is first compiled, and then all of its parts are packaged into one file. An APK file contains all of that program's code (such as .dex files), resources, assets, certificates, and manifest file. As is the case with many file formats, APK files can have any name needed, provided that the file name ends in ".apk".
res: the directory containing resources not compiled into resources.arsc (see below).
assets: a directory containing applications assets, which can be retrieved by AssetManager.
AndroidManifest.xml: An additional Android manifest file, describing the name, version, access rights, referenced library files for the application. This file may be in Android binary XML that can be converted into human-readable plaintext XML with tools such as AXMLPrinter2, apktool, or Androguard.