Android application package (APK) is the package file format used by the Android operating system for distribution and installation of mobile apps and middleware.
Why would I want to install an APK File? Searching for and downloading APK files allows you to access apps that have leaked ahead of their official release. (You can also download apps which have been banned, though we would not recommend this.) APK files of new Android builds are often leaked ahead of time, giving you early access to all the exciting new features that otherwise wouldn’t be available to you. A recent example of this would be with the early Android L build which found its way online despite that it’s yet to be officially released.
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. By default, the ability to install from unofficial sites or directly from a desktop is disabled for security reasons on most Android devices. You can enable it by changing the setting “Unknown sources” in the Settings menu.
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.