you might find a slide set I wrote for a lecture last year helpful:
http://pumptheory.com/collaboration/open-acs/lectures/openacs-workflow/
also, the original design spec for workflow:
http://www.collaboraid.biz/developer/workflow-spec
Also you can (inconveniently) browse the docs for an up to date workflow package via the cvs browser:
http://cvs.openacs.org/cvs/openacs-4/packages/workflow/www/doc/
eg: the developer docs:
http://cvs.openacs.org/cvs/*checkout*/openacs-4/packages/workflow/www/doc/developer-guide.html
