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The architecture of the Jabber network is similar to email while a Jabber server conforms an email server (server.com) and Jabber clients conforms email clients ([email protected]). The difference is that everything happens (by definition: nearly) in realtime over a persistent XML-TCP stream. In example this makes it possible that other (authorized) users can see your online presence. In addition to that servers can offer specialised services, in example a user directory (users.server.com) or a weather agent (weather.server.com). XMPP is the architecture/protocol the network is based on.
E especificamente:
What is the difference between Jabber and XMPP? Since 2004 XMPP is the IETF-approved instant messaging and presence protocol. XMPP is based on the Jabber protocol that evolves since 1998. Basically programming Jabber software today means programming XMPP compliant software.