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Instant Messaging (IM)
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XMPP / Jabber protocol
XMPP / Jabber server

Concepts bullet XMPP / Jabber protocol

What is XMPP? What is jabber?

Extensible Messaging and Presence Protocol (XMPP) is an open-standard communications protocol for message-oriented middleware based on Extensible Markup Language (XML). Early on, this protocol was called jabber and it was meant for near-real-time, extensible instant messaging (IM), presence information, and contact list maintenance. Designed to be extensible, the protocol today also finds application in Voice over Internet Protocol and file transfer signaling.

References:
http://searchnetworking.techtarget.com/definition/Jabber
http://archive.jabber.org/userguide/