Google Talk is Google's new IM (Instant Messenger) service that is now available. The new service allows you to call or send IM messages to your friends around the world. Other features include your Gmail contacts are also automatically pre-loaded into Google Talk.
Google is committed to using open standards, that is why they support the same open protocols that other IM clients use such as Trillian, GAIM, and iChat do. For right now, if you want to talk to your friends on ICQ, Yahoo, or MSN with the new Google Talk client its not supported at this time.
Here are some resources to answer your questions about Google Talk: