From http://wiki.xmpp.org/web/Jabber_Programmer%27s_Guide
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
(user@server.com). 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.
In other words, XMPP is the protocol, Jabber is the name of an implementation, specifically for instant messaging. In an instant message program context, they are essentially the same thing.