You might first become aware of the philosophy and goals of XForms. If this is the case, then this article from IBM can help you:
www.ibm.com/developerworks/library/x-xformswhy.html

Then, you need a more technical guide. Intalio uses Orbeon's XForms server. Orbeon provides good information that includes some technical explanation and an example for each XForm control:
www.orbeon.com/ops/doc/reference-xforms

If you're looking for more tutorial-style guide, then go to:
Wikipedia: en.wikipedia.org/wiki/XForms
or
IBM - tutorial www.ibm.com/developerworks/library/x-xformswhy.html

However, if you really need to go to the source, the technical standard, then go to W3C:
http://www.w3.org/TR/xforms/

Finally, when you have everything ready, you should create some architecture that uses Orbeon's server as the XForms renderer. This requires going through the Orbeon's website and documentation.
First, a short tutorial (very Orbeon-specific):
http://ogres.sandre.eaufrance.fr/ops/doc/intro-tutorial

Then, how to install: http://ogres.sandre.eaufrance.fr/ops/doc/intro-install

Then, how to integrate it to Java apps:
http://ogres.sandre.eaufrance.fr/ops/doc/reference-xforms-java

I hope this information is useful !
Cheers,

Jdlacruz