.APK is the installer file extension of applications in android operating system like .exe in Windows. If you have .apk file of an application, you can install the app without accessing the Google play store. Even you can install it without internet connection and can send to your friends via bluetooth. An APK installer file is usefull when Google Play says your device is not compatible with the App. or that particular App is not available in your country. Downloading and keeping an APK installer file is a way get access to older versions of that app (Google Play store serves only the latest App versions).
Download APK installer of Android Apps
First you need to access Google Play store via a web browser (Not through the Play Store App on the phone). and find your App by searching it, and copy the url of the App page. The url of the WhatsApp application for Android may look line this for me https://play.google.com/store/apps/details?id=com.whatsapp&hl=en . Now go to Apps.evozi.com and paste the Application url and click on “Generate Download link”. Now the tool will access the Google Play store and fetch the APK file and you can download setup file from google play to your Windows PC, Mac, Android smartphone, Tablet, etc. You can download only free apps’ APK files with this method, cannot download paid Android app APKs.