phranque - 4:23 am on Sep 14, 2012 (gmt 0)

menus are used for creating a list or multi-level tree structure for navigation.
categories are a way of organizing articles.
a module is a reusable subset of a page's content that can be assigned to a named position in a template.

maybe this will help...

Understanding sections, categories and articles - Joomla! Documentation:
http://docs.joomla.org/Understanding_sections,_categories_and_articles [docs.joomla.org]