Facebook owned company, WhatsApp, known to be a mobile only optimized app can now be used on a desktop. The desktop version is simply an extension of the Phone App, which currently boasts over 500 million users worldwide!
To use the WhatsApp web based client, you need to connect the WhatsApp on your phone to that of the website by pairing & scanning the QR code on your phone with the web version. This is because the phone WhatsApp uses a phone number for user verification as opposed to a user name and password.
Android, Blackberry and Windows phones (with the exception of iOS) already have the new WhatsApp web client working smoothly.