This might be an old question, but I just wanted to keep the process I used in order to learn XMPP.
A few years ago, a few friends of mine and I were learning about how to leverage XMPP, and understanding how it fits into larger piece is quite a tedious task. I highly recommend starting off by reading the wikipedia page of XMPP:
http://en.wikipedia.org/wiki/XMPP
You'll be surprised how many people aren't able to answer questions about XMPP which are the most fundamental.
I also highly recommend reading this article:
http://www.infoworld.com/article/2682116/application-development/xmpp-rises-to-face-simple-standard.html
It'll give you a sense of the motivation behind XMPP, it's history, and it's protocols that used to be on par with it.
From there, it'll be best to read the sources of the wikipedia page to give a more indept understanding of any features you might be interested in with XMPP.