Google today began rolling out Google+ Hangouts functionality to Gmail, replacing (upgrading) old Gmail video chat. The biggest difference between two is that unlike old video chat, which was based on peer-to-peer technology, the upgraded service will offer higher reliability and enhanced quality by utilizing the power of Google's network.
According to Google, this upgrade will benefit all the Gmail users but if you and the person you're chatting with also use Google+, you’ll get even more from the Hangouts experience. And the coolest part is you'll be able to video chat with up to nine people at once, watch YouTube videos together, collaborate on Google documents and share your screen.
Source:- Official Gmail blog