WhatsApp Web is an app that supports all operating systems. You can install it on your computer in order to access your WhatsApp account not only from your phone. After the installation you will have to connect it to the WhatsApp on your phone. Using WhatsApp Web on desktop is only possible when your phone is connected to the internet.
First, go to WhatsApp Web domain on your PC and download the relevant version for Mac or Windows. After downloading, open it. The QR code should pop-up.
Secondly, in the WhatsApp menu on your phone, go to settings and look for “WhatsApp web”
Thirdly you will be asked to scan the QR code from the desktop application in order to establish connection.
After you scan it, everything should be transfered to the desktop, from which you will now be able to access all the chats, groups. You can only access WhatsApp on your computer if its in constant connection with your phone, all the chats are storred on the mobile, and the desktop version solely enables sending and receiving messages, pictures, GIFs. There is no possibility to call using WhatsApp desktop.