WhatsApp Web is not another WhatsApp account. When you use WhatsApp on your computer and your phone, you are simply accessing the same account on these two devices.
There are a few minimum requirements to enjoy WhatsApp Web:
- You need to have an active WhatsApp account on your phone.
- You need to have a stable internet connection on both your phone and your computer.
- You need to use the latest version of Google Chrome, Mozilla Firefox, or Opera as your web browser.
- Visit web.whatsapp.com on your computer.
- Open WhatsApp on your phone and go to Menu > WhatsApp Web.
- Scan the QR Code on your computer.
NOTE: To avoid data usage charges on your phone, we recommend that you are always connected to Wi-Fi when using WhatsApp Web.
Source : http://www.whatsapp.com