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 a Debian package in Debian-based operating system. To make an APK file, a program for Android is first compiled, and then all of its parts are packaged into one container file. An APK file contains all of a 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 the file extension “.apk”.
APK files are a type of archive file, specifically in zip format-type packages, based on the JAR file format, with .apk as the filename extension. The MIME type associated with APK files is application/vnd.android.package-archive.
APK files can be installed on Android-powered devices just like installing software on a PC. When a user downloads and installs an Android application, from either an official source (such as the Google Play Store), or from an unofficial site, they are installing an APK file on to 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. The installation of APK files downloaded outside the Google Play is disabled by default. Users can install unknown APK files by enabling “Unknown sources” from “Accounts and Security” in Settings.