XMPP

Definitions of XMPP on the Web:

• What is XMPP?
  
  XMPP (or the eXtensible Messaging and Presence Protocol) is an IETF standard for interoperable instant messaging and presence.

• Short for Extensible Messaging and Presence Protocol, an open, XML-based protocol for server-to-server near-real-time extensible instant messaging and presence. XMPP is a rival protocol to SIMPLE.

  XMPP also is known as the Jabber protocol because Jabber is based on XMPP. Jabber is not, however, the only instant messaging application that relies on XMPP.

• Extensible Messaging and Presence Protocol, or XMPP, is an open, XML-based protocol for near real-time extensible messaging and presence events. It is the core protocol of the Jabber Instant Messaging and Presence technology which is currently deployed on thousands of Jabber servers across the Internet and is used by millions of people worldwide. The Internet Engineering Task Force (IETF) XMPP working group aims to adapt the XMPP for use as an IETF Instant Messaging and presence technology.
  en.wikipedia.org/wiki/XMPP

An important article on XMPP and Google Talk

Google Talk gives boost to XMPP Once an underdog in the IM standards race, XMPP gains backing from the Web-search giant in a move that may have far-reaching impact.
http://www.infoworld.com/article/05/08/26/HNgooglexmpp_1.html