Till now WhatsApp worked for Android, iOS, Blackberry, and Windows Phone.
“Like WhatsApp Web, our desktop app is simply an extension of your phone: the app mirrors conversations and messages from your mobile device. Put another way: you need the mobile app running on your phone, close to your desktop computer, for it to work.”
To use WhatsApp on your desktop you need to log in to the app, scan a QR code using the WhatsApp mobile app. After that all your chats and conversations will be loaded.
WhatsApp has more than 1 billion users already. As of today, they have one more reason to keep it open all day.