The search giant Google are doing a great job of a quietly merging all of its user services into one place, and therefore making it easier to access these features from one page. In this case its Google+ Hangouts that has been made available in Gmail, it replaces the traditional video feature that relied on outside technology, no its on the Google network and should be a smooth as glass to use.
According to the posting by Fred Brewin, Product Manager on the Official Gmail Blog;
All Gmail users will benefit from this upgrade, but if you and the person you’re chatting with also use Google+, you’ll get even more from the Hangouts experience. 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. Plus, Hangouts has a bunch of fun effects that you can try out…because any conversation is better when you’re wearing a virtual mustache or pirate hat.
Source [Gmail Blog]