Google Chat used to be usable within Gmail, Orkut and iGoogle but the last two services were shut down, so presently Google Chat can be used only within Gmail.
At this time there are two alternatives for chatting within the GMail web interface: the "old chat" (Google Chat) and Hangouts. Each has their own help centre, a different user interface and some distinctive features, like Hangouts in Chrome doesn't require a plug-in for audio and video conversations, among other features.
The gmail-chat doesn't have information in its wiki tag page. Previously, the google-chat and google-talk were merged, as I understand this, because in the context of Web Applications they refer to the same service.