Google’s instant messaging service launched in 2005 and later replaced by Hangouts was Google Talk. It supported text messaging and, over time, voice and video communication.
Google officially launched Google Talk on August 24, 2005. The service used the open XMPP messaging protocol, which meant users could connect through Google’s own software as well as some compatible third-party clients. It was closely integrated with Gmail and became widely known informally as Gchat or GTalk.
Google expanded Talk’s capabilities over the following years, including computer-to-computer voice and video chat. In 2013, Google announced that Google Talk, Google+ Messenger, and the original Google+ Hangouts video service would be replaced by Google+ Hangouts.
Allo and Google Voice were separate Google products, not the service that directly replaced Google Talk. Google Chat is also easy to confuse with it, but Google Chat was introduced later and eventually became part of Google’s successor messaging lineup after Hangouts was retired.