Hangouts allows conversations between two or more users. The service can be accessed online through the Gmail or Google+ websites, or through mobile apps available for Android and iOS (which were distributed as a successor to their existing Google Talk apps). However, because it uses a proprietary protocol instead of the XMPP open standard protocol used by Google Talk, most third-party applications which had access to Google Talk do not have access to Google+ Hangouts.
Forrás: https://en.wikipedia.org/wiki/Google_Hangouts#Features