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.
There are many sources of APK files across the web, but be sure to choose a site which you trust. Some APK files may contain malicious software (malware) which could compromise your phones security, so it is best to be cautious before downloading or installing any app from outside of the Play Store (and even in the Play Store you’re not completely safe!). Generally speaking, APK files found on sites like XDA should be safer to download than from elsewhere, but read some reviews and user comments before downloading or installing an APK.
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.