Its currently available for Android, Windows, Blackberry. But not for iOS devices currently. Here you can send and receive messages from your computer.
Just open the WhatsApp web and scan the QR code given on website. At the time of scanning your smartphone will pair the web browser to your phone and its give the output of all your conversation messages. This will happen when your phone is connected to internet.
WhatsApp Inc owned by Facebook. In October 2014, 600 million active users are on WhatsApp globally and in January 2015 its reported with 700 million active users globally.
Press Release:
"Today, for the first time, millions of you will have the ability to use WhatsApp on your web browser. Our web client is simply an extension of your phone: the web browser mirrors conversations and messages from your mobile device -- this means all of your messages still live on your phone.Source: WhatsApp Web
To connect your web browser to your WhatsApp client, simply open https://web.whatsapp.com in your Google Chrome browser. You will see a QR code --- scan the code inside of WhatsApp, and you’re ready to go. You have now paired WhatsApp on your phone with the WhatsApp web client. Your phone needs to stay connected to the internet for our web client to work, and please make sure to install the latest version of WhatsApp on your phone. Unfortunately for now, we will not be able to provide web client to our iOS users due to Apple platform limitations.
We really hope you find web client useful in your everyday lives."