Android application 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 .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”.
First of all, we will have to enable the option to install the applications from unknown source on our Android. Open your Android system settings and click on Applications (Programs for some of the HTC devices) and check the option Unknown source. Those of you who are on Android ICS 4.0 you will find these settings under the security settings. On most of the devices, you can now simply click on the file using any of the file managers to install them, using the built-in package manager. If your device does not have a package installer by default, you can download and install an APK installer application from Google Play.
When you want to install an APK manually, just open the APK installer app and navigate to the folder where you have your APK files saved. Now, simply click on the application to bring on the installation screen. Click the install button to successfully install the application.