At the last Gartner BPM Summit I gave an overview presentation on BPMN, XPDL, BPEL, and Wf-XML. Due to the positive response from the presentation, I was asked to record it so it could be accessed from the web. This is available and can be accessed by clicking on the link below. There are a few slides from my employer up front, please understand they support all my time and effort to make this information available to everyone for free, so after a few slides on the Fujitsu Interstage product, you will get to a fairly succinct overview of these four important standards.
It runs about 30 minutes total. Enjoy.