APK

Android Package (APK) is the package file format used by the Android operating system for distribution and installation of mobile apps and middleware.

APK files are analogous to other software packages such as APPX in Microsoft Windows or Deb packages 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 .dexfiles), 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".

APK files are a type of archive file, specifically in zip format packages based on the JAR file format, with   as the filename extension. The MIME type associated with APK files is .

APK files can be installed on Android powered devices just like installing software on PC. When a user downloads and installs an Android application from either an official source (such as Google Play), or from some other (unofficial) site, they are installing an APK file on their device. A user or developer can also install an APK file directly to a device (that is, not via download from the network) from a desktop computer, using a communication program such as adb, or from within a file manager app in a process known as sideloading. By default, the ability to install from unofficial sites or directly from a desktop or file manager is disabled for security reasons on most Android devices. Users can enable it by changing the setting "Unknown sources" in the Settings menu.

Package contents
An APK file is an archive that usually contains the following files and directories: Signature-Version: 1.0 Created-By: 1.0 (Android) SHA1-Digest-Manifest: wxqnEAI0UA5nO5QJ8CGMwjkGGWE= ... Name: res/layout/exchange_component_back_bottom.xml SHA1-Digest: eACjMjESj7Zkf0cBFTZ0nqWrt7w= Name: res/drawable-hdpi/icon.png SHA1-Digest: DGEqylP8W0n0iV/ZzBx3MW0WGCA=
 * directory:
 * : the Manifest file
 * : The certificate of the application.
 * : The list of resources and SHA-1 digest of the corresponding lines in the MANIFEST.MF file; for example:
 * : the directory containing the compiled code that is specific to a software layer of a processor, the directory is split into more directories within it:
 * : compiled code for all ARM based processors only
 * : compiled code for all ARMv7 and above based processors only
 * : compiled code for all ARMv8 arm64 and above based processors only[7][8]
 * : compiled code for x86 processors only
 * : compiled code for x86 64 processors only
 * : compiled code for MIPS processors only
 * : the directory containing resources not compiled into resources.arsc (see below).
 * : a directory containing applications assets, which can be retrieved by .
 * : 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.
 * : The classes compiled in the dex file format understandable by the Dalvik virtual machine and by the Android Runtime.
 * : a file containing precompiled resources, such as binary XML for example.