hank5678 wrote: Does Community Builder use and extend the Joomla user database or does it just sync the Joomla user data with it's own database?
This info is relevant to help you understand better how CB interfaces with Joomla:
www.joomlapolis.com/downloads/downloads-single-items/doc_download/225-cb-api-document-draft-006
Answer is that CB updates and uses the Joomla user creation API, but keeps extra info/data in its own tables. So if you remove CB in the future your users will still be in the Joomla user database.
You can of course test this out as CB is free to download.