All articles and library items belong to a category. Categories are specific to each type of item - article, file, book etc..
Categories are arranged in a hierarchy, with a main category (eg Articles - a type of content) having a number of sub-Categories (eg Articles / People for articles about specific individuals). Because an item can only belong to one category, the category hierarchy is not very deep. Tags provide a more useful way of organising material.
The current top level categories which have their own hierarchies are Articles, Documents, People, and Books.
As well as a category, an item can have any number of tags assigned (usually no more than five or six. Some tags apply across multiple types of item (eg Themes or Organisations), others may be specific to one type of item (eg Newsletter or Pamphlet). An article might reference several different organisations, so may have more than one organisation tag, or a document might serve as both both a flyer and a poster, or contain both minutes and an agenda.
Tags are grouped together in different types of tag. The current top level groups include Doc.Types, Article Subjects, Themes, Places, People, Organisations & Groups, Events,
You can see the hierarchies for both categories and tags in sidebar modules on the right hand side of this page. Other pages have similar lists in the sidebar. Click on the title for a list of all items belonging to the category or having the tag.