what I know on jabber:
jabber addresses look like e-mail addresses: name@server. if you are registered on the jabber.org server , you might be called firstname.lastname@example.org (well, actually not, since that's me). Gaim an probably all jabber clients allow you to create an account on open servers. You specify the server, the desired name, server and password and Gaim tried to register you with the specified server.
People on different servers can inter-communicate. In jabber, you will often see addresses in the form name@server/resource . The default "resource" for Gaim users is "Gaim", but this can be changed. This allows you to log in multiple times. This has been used to have scripts connect to the jabber network without having a seperate account (there are modules for most major scripting lingos)
* an instant messenger with many advanced features,
* encrypted using proven, industry-standard methods,
* available for Windows, Mac OS X and Linux,
* direct connect, sending messages, etc. without a server,
* its own network that does not rely on another system,
* committed to privacy with no ads or spyware.