If you have an Android device, you will likely have heard the term APK and wondered what it means. Let’s quickly discuss what an APK is and why it is important to Android.
What Is APK?
Basically APK stands for Android Package Kit (APK)  (also Android Application Package) and it is actually an extension use by the Android operating system for distribution and installation of mobile apps and middleware, In APK have all the elements that an Android application need to install correctly on your device.
Just like the EXE Software Extention on Windows, you can place an APK file on your android device to install the APK App, Manually installing apps using APKs is called sideloading.
Normally when we visit Google play store to download an app,  First it automatically downloads the APK App and then installs it, when installation complete it automatically removes the APK App file from your android device.
How to install  APK from your Android device
To Install APK app on your android smartphone You need to make these changes in your Andriod devoice setting,
For Android OS version 2.3 and earlier:
* 1 – Go to Settings.
* 2 – Go to Applications.
* 3 – Tap and check the 'Unknown sources' box.
* 4 – Tap on Ok when it shows the warning.
That's it, how you can enable the installation of APK files on your phone manually.

For Android OS version 4.0+:
* 1 – Go to Settings.
* 2 – Go to Security.
* 3 – Scroll down and check the 'Unknown sources' box.
* 4 – Tap on Ok when it shows the warning.
All Done,