A few months ago WhatsApp announced the official Chrome app for WhatsApp using which anybody can use their WhatsApp account through the Chrome browser. While this is very convenient, you have to keep your Chrome browser always open to receive WhatsApp chats notifications and such. This obviously leaves out those people who do not use the popular Google Chrome browser and are fond of some other web browser like Firefox or Opera. All these people can now install a third-party WhatsApp Desktop Client to enjoy WhatsApp on their desktop PC.
After the installation of this WhatsApp desktop client developed by Alexandru Rosianu, you can launch it and it will automatically show you the QR code that should be scanned through the WhatsApp mobile app (in your Android or iPhone devices). To scan this QR code, you can open the menu in WhatsApp mobile app and choose WhatsApp Web from there to start the QR code scanner.
As soon as the QR code is scanned, the WhatsApp desktop client loads your WhatsApp account and allows you to chat with your contacts. You can do everything that you can do on the WhatsApp mobile app. You can even receive notifications in the notification area of Windows (and equivalent places in other operating systems).
Actually this WhatsApp desktop client uses a portable version of the Chromium browser customized to look like an ordinary Windows application with no address bar, toolbar etc. This portable browser is opening the website https://web.whatsapp.com/ which is same as the using the official WhatsApp web app in your Chrome browser.
You can download WhatsApp desktop client from http://whatsapp-desktop.com/.