Google Talk was an instant messaging service that provided both text and voice communication. The instant messaging service was variously referred to colloquially as Gchat, Gtalk, or Gmessage among its users.
Google Talk was also the name of the client applications previously offered by Google to use the service. Google Talk applications were available for Microsoft Windows, Android, BlackBerry OS, BlackBerry 10 and ChromeOS operating systems. A Google Talk mobile web app had also been previously available. In February 2015, the Windows client was discontinued and ceased to work, with Google recommending users to use Google Hangouts instead. Users of Windows client were instructed to migrate to the Google Hangouts app on the Chrome browser platform. Currently, Google is migrating its users from Google Hangouts, to Google Chat and Google Meet.
Features
Product integration
Google Talk was integrated into Gmail where users could send instant messages to other Gmail users. As it worked within a browser, the Google Talk client did not need to be downloaded to send instant messages to Gmail users.
Conversation logs were automatically saved to a "Chats" area in the user's Gmail account. This allowed users to search their chat logs and have them centrally stored in their Gmail accounts. For a long time, it was not possible to directly download chat logs that were not attached to an email conversation, although some workarounds had been found. However, on September 15, 2011, Google announced a new feature of its Google Takeout program that allows users to download chat logs via IMAP.
Google also integrated Google Talk with Orkut. This enabled Google Talk users to interact with registered Orkut users, by sending and receiving "scraps" within Orkut. Orkut has since been shut down.
Google Talk Gadget was a web-based module that could be embedded in iGoogle (since discontinued) and other web pages, allowing text chat with users of Google Talk. This functionality was at some point discontinued without any real announcement, likely in the face of the release of the Google Hangouts services.
Google+ was integrated into Google Talk. In the standalone client and the Google Talk widget embedded into Gmail and Google+, Google+ contacts appeared in the contacts list; their respective circles were shown in Google+'s iteration of the widget. Google+ has since been shut down.
Voice and Video
It was possible to place and receive phone calls from within Gmail by using Google Talk. In order to receive calls, however, the user had to upgrade to a full Google Voice account. Initially, users outside of the US could not upgrade to a full Google Voice account or receive phone calls in Gmail. (Outbound calling through Gmail did not require Google Voice and was available in many countries outside the US.) Google Talk allowed users to leave a voicemail for a contact whether or not they were signed into Google Talk. Messages could be up to 10 minutes long and users could choose to have them sent to their Gmail inbox. Messages could be sent with or without first ringing the recipient's phone number.
Google also provided a Voice and Video Chat browser plugin for Google Talk (not to be confused with the standalone Google Talk client) that supported voice and video chat between Gmail users. The plugin was available for Windows (XP, Vista, and 7), Mac OS X (only on Intel-based Macs), and Linux (Debian, Ubuntu, Fedora, and OpenSUSE packages available, but the binaries worked on other systems). The plugin had to be downloaded and installed, but otherwise seamlessly integrated into the Gmail interface.
Encryption
The connection between the Google Talk client and the Google Talk server was encrypted, except when using Gmail's chat over HTTP, a federated network that didn't support encryption, or when using a proxy like IMLogic. End-to-end messages were unencrypted. Some XMPP clients natively supported encryption with Google Talk's servers. It was possible to have end-to-end encryption over the Google Talk network using OTR (off-the-record) encryption using other chat clients like Adium (for Mac) or Pidgin (for Linux and Windows).
Google's version of "Off the Record" was not OTR (off-the-record) encryption. Enabling "off the record" inside Gmail's Chat turned off logging of messages, but did not enable encryption.