https://www.whatsapp.com/download/
Most persons should already know about
WhatsApp Web. This allows you to use WhatsApp from your browser. Today when I signed in to that I noticed a link saying a desktop app is now available for both PC and MAC. Testing now. Will edit back and say what's up with it.
*EDIT*
So it seems to be nothing more than a dedicated app which does the same thing as the web interface. Not seeing any real benefit to having it over using the web interface. So up to you if you want to install an app for that or just use your browser IMO. And I'm seeing posts online from 2016, so this comes in very late. But just so we have the info here.