The first match on http://www.google.com/search?q=jabber clearly explains what Jabber is-- in fact, the very next match is conveniently titled "What Is Jabber?"
Anyway, to the OP:
Google would probably help you as well, more than we can anyway:
Just to add onto this, IRCap contains a jabber script. I'm not sure if the author allows it to be used outside of IRCap but it could still be handy to learn from.