Hangouts is a newly released mobile app from Google. It was announced at Google I/O 2013, and replaced several applications such as Google Talk and Google+ Messenger. Hangouts is able to connect to Google's other chatting services, including Google+ Hangouts, and any other place you're able to have a voice or video chat.
Hangouts was Google's solution to an issue they had with their chatting services. Before hangouts were released, Google had multiple mobile applications that all did the same thing. Hangouts was able to replace them, and simplify the mobile experience.