You will find the specific details of individual plugins/widgets for the Wordpress theme and how to use them under each theme and plugin. Plugins are categorized by whether they are Free (also available through the wordpress.org plugins site), Premium (requires Semiologic Pro subscription) or Retired (no longer supported not maintained).
On occasion several plugins may need to be used together, some other circumstance may be required for a plugin to function correctly or a concept may need to be more fully explained. In this case you may find a more general discussion in the Resources area.
There may well be some duplication and overlap between the two areas (as, indeed, you will find this explanation in both).