What is .apk file android ? 

apk refers to "Android Application Package". It is the file format used by android to distribute and install android software and middleware onto Google's Android Operating System. If you want to install an application on Windows, we have an .exe file ,for Mac OS X, we have .dmg files just like that for android we have .apk file.

In more advance apk files are ZIP file formatted package based on JAR file format. If you install android software apart from Google Play store you have to  allow apps from unknown sources to install that apk file.

How to install .apk files ? 

When you install applications from Google Play Store you just click "install". Everything is automated. You had to wait till it complete installing the application. That's all. So why .apk files.  Google Play Store doesn't have all the android applications we want. So we had to depend on  authorized websites to download them. When you download such software you will get the .apk file. You have to install this application manually. As I mentioned, the apps come as APK files so that they could be installed on an Android device. Before installing the APK file you had make some arrangements.

Step 1: Enable "unknown sources" option on your Android.

Step 2: Find .apk file using your favorite file manager and click to install

Then Enjoy most awesome apps