In 2008 Google introduced video chat from Gmail and now they are adding Hangout feature of Google+ in the Gmail.
Previously video chat was based on peer-to-peer (single person to single person) technology. Now with this new feature you can reach people on your Google+ list. You can also chat with those who are using their Google account through Android or iOS based Smartphone or Tablet from your Gmail account itself. Official video of this feature can be watched to understand what this really means.
You can start video chat with up to nine people at once, watch YouTube videos together, collaborate on Google documents and share your screen (take screenshot and send) with those who are on Android and iOS devices from Gmail inbox. This upgrade will be beneficial for the all Gmail users but if you and the person you're chatting with also use Google+, you'll get even more from the Hangouts experience. Bunch of fun effects are available in the hangoutthat you can try out, like pirate hat or mustache. Gtalk Android application already have this feature , but the recent step brings this functionality to a wider number of devices so that one needs not be bothered about which device his/her friend is on.
Source : Themobileindian