Let users categorize content into a hierarchal section. Allow users to select a hierichal, and possibly nested, category for their content that matches the hierichal categorization of the site itself.
Where tagging works to explain tiny distinctions and details in content, categories represent broader and more easily explained distinctions.
Having multiple categories help to wall section off from each other and help suggest what content is to be found, needed and appropriate for a site.
Categorization is the process in which ideas and objects are recognized, differentiated, and understood1. Categories help us make sense of the world faster and more easily. We seek to categorize all we experience in an attempt to explain how the world works – how our knowledge is represented in the real world.