from http://www.jabber.org:
Jabber is a set of streaming XML protocols and technologies that enable any two entities on the Internet to exchange messages, presence, and other structured information in close to real time. The first Jabber application is an instant messaging (IM) network that offers functionality similar to legacy IM services such as AIM, ICQ, MSN, and Yahoo. However, Jabber is more than just IM, and Jabber technologies offer several key advantages ...
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Basically it's an Open Messaging System. It's too bad more people don't use it. A great feature is it's ability (through "transports") to send messages to other IM systems (e.g. AIM, MSN, etc).