Check here for configuration.
Basically, if you have a Gmail account and a Jabber or compatible messaging client, you can check it out. I'm online, so stop and say hello!
The Google Talk program has it's own interface, which can be downloaded here. It automatically picks up your most commonly emailed contact in Gmail and asks you who you'd like to invite. Users not set up with a Gmail address are sent an invitation.
Some current features:
It's free Simple and easy-to-use interface No pop-ups, ads or other unneccessary "fillers" like Y! and others "Regular" instant messaging VoIP (speak realtime using speakers and a mic) Address book from Gmail auto-loads your friends - you choose your contacts from a pre-loaded list or add your own. Can be accessed through other multi-client messengers, like Trillian. Chat-logging
The interface couldn't be simpler, so it's easy to set up and get started. Lots more features are being worked on. This is only the early stages, and they do plan to open up the ability to use Google Talk without having a Gmail account. EarthLink and Sipphone are only two of the additional providers they're currently working with to expand to this service.
If you do try it out, look me up! My Gmail address is provided in the upper right.