The Download Now link directs you to the Windows Store, where you can continue the download process. You must have an active Microsoft account to download the application. This download may not be available in some countries.
What's Web is a dedicated view for the WhatsApp Web service. Now you can use WhatsApp on your computer without accidentally disconnecting when closing your web browser. An active WhatsApp account is required to use WhatsApp web. Your phone WhatsApp app is required to login to the web service. Both your phone and this app require an active internet connection. Due to some limitations not all functions of WhatsApp web might be available, however the chat function is fully available. This is not an official WhatsApp product. I'm just a WhatsApp user who build this app for personal use and willing to share it with the community.