Instant messaging between the Google Talk servers and its clients uses an open protocol, XMPP, allowing users of other XMPP/Jabber clients to communicate with Google Talk users. VoIP in Google Talk uses an older version of what would later become the Jingle protocol. The technology used within the Google server network however is not publicly known.

The Google Talk client is only available for Microsoft Windows (2000, XP, Server 2003, Vista and Windows 7), but Google Chat (voice and video chat) is now available for PCs and Intel-based Macs. Google Talk mobile clients are also available for the Palm Pre, BlackBerry, iPhone, Android-based devices and is preloaded on the Nokia N900 smartphone. With the release of the Google Talk gadget, users of all platforms supported by Adobe Flash Player can also use Google Talk. Many other XMPP clients are compatible with Google Talk, and support a variety of other platforms.

Google has announced that a major goal of the Google Talk service is interoperability. Google Talk uses XMPP to provide real-time extensible messaging and presence events, including offline messaging and voice mailing. On January 17, 2006, Google enabled server-to-server communications, federating itself with any XMPP server that supports the dialback protocol.

On February 7, 2005, Gmail received chat functionality, using Ajax for server–browser communication, and was integrated with Google Talk. Users can send instant messages to other Gmail users. As it works within a browser, Google Talk does not need to be downloaded to send instant messages to Gmail users.

Conversation logs are automatically saved to a Chats area in the user's Gmail account. This allows users to search their chat logs and have them centrally stored in their Gmail accounts. It does not, however, appear possible to download chat logs that are not attached to an e-mail conversation.

Since November 8, 2006, Google has also integrated Google Talk with Orkut. This enables Google Talk users to interact with registered Orkut users, by sending and receiving 'scraps' within Orkut.

It is also possible to display the song currently playing on one's computer.

Also, as of November 11, 2008, the new Google Chat supports voice and video chat between Gmail users and Google Talk users—on some systems, not all major OSs are supported. The Gmail user requires a plugin download and installation, but is otherwise seamlessly integrated into the Gmail interface. The plugin is available for Windows (XP, Vista, and 7), Mac OS X (only on Intel-based Macs), and Linux

The connection between the Google Talk client and the Google Talk server is encrypted, except when using the international version of the client, Gmail's chat over HTTP, a federated network that doesn't support encryption, or when using a proxy like IMLogic. Thus messages are not necessarily encrypted end-to-end, although it is possible to have end-to-end encryption over the GTalk network using Zfone or OTR (off-the-record) chat when all participants connect over HTTPS. Some XMPP clients also natively support encryption with Google Talk's servers.

On July 28, 2006, Google added voicemail and file sending capabilities to the Google Talk client. Voicemail messages can be 10 minutes long, and they're delivered to the contact's mailbox as an attached MP3 file (11 kHz mono 24kbit/s). Recipients who use Gmail are offered better integration. Gmail recognizes that it is a voicemail message, and users can choose to stream the file using the integrated MP3-playing applet, or to download the MP3 file.