Ok I tried going to jabbers site, and it looks like cisco has acquired them.
Is jabber a server or a client? is it open source?
From what I guess, the steps are:
Jabber is the same thing as XMPP. The canonical place to find out about jabber is www.xmpp.org. They host all the documentation about the protocol. If you just want to use it, jabber.org hosts a free jabber server you can connect to, and is a great place to try it out. Psi (psi-im.org) is a good free client to try out the system.
The jabber protocol is a bit like smtp for instant messaging (IM). Servers federate (talk to each other) and the protocol is plain text, and defined in several RFCs. There are both client and server pieces of the protocol.
As you discovered, jabber.com (Jabber, Inc) was purchsed by Cisco. Jabber, Inc sold a proprietary jabber server (XCP) with great scalability that used the defined RFC protocols. There are several other servers out there, some free, some not. They all can communicate, which is the cool thing. Or not communicate, which is also cool.
I recommend trying out jabber clients, finding an active chat room (called MUC, multi user chat) and see where that takes you. If you're a developer you can use XMPP for any presence enabled thing. It's very extensible.
Oh, if you've used gtalk, you've used jabber. It's popular, open, and not tied to any one particular company.