I think you can summarize the difference pretty succinctly as UDDI being a specific enabling technology (which Web Services can make use of) and ebXML being a broad, strategic architecture for eBusiness interactions. I actually found a great ebXML overview book "ebXML: The New Global Standard for Doing Business on the Internet" (that you can occasional find at discount these days) if you wanted a quick read/overview.
Paul Monday
Author, Web Service Design
Patterns:
Java Edition