WhatsApp has released an official web app for the desktop users. Prior to this people had to use BlueStacks, AirDrioid's AirMirror or PushBullets (for notifications) but all these third party tools were inconvenient. Now you can just go to https://web.whatsapp.com/, scan the QR code with the WhatsApp mobile app, and start using WhatApp on your web browser.
This is as simple as it can be.