The new WhatsApp desktop application is available for Windows 8+ and Mac OS 10.9+ and the desktop application will be synced to your mobile device.
This means that the desktop application is an extension of your phone’s account and the app will have the same functionality showing the same messages and conversations from your mobile device on desktop. It also means that it will only be active if the WhatsApp on your mobile device is on, ie if there is no data network on your mobile account then you cannot receive or send messages using the desktop application.
To download the desktop application follow this link https://www.whatsapp.com/download/
And after downloading the app and launching it you need to scan the QR code by selecting the WhatsApp Web option on your WhatsApp mobile account.