While video chatting in Gmail isn't new, Google has decided to upgrade that function with Google+ Hangouts. According to Google, "Hangouts utilize the power of Google’s network to deliver higher reliability and enhanced quality". Which means you can expect better video calls with all those people you have been video chatting with in Gmail, and you can start a Hangout if both of you are using Google+. So chatting with nine people simultaneously, collaborating on Google documents and watching YouTube videos together is now possible straight from your Gmail. In fact, the chat function isn't only limited to Gmail, you can even chat with friends who are using Google+ on their web browser, iOS or Android device.
Source: Official Gmail Blog