Categories are used in Wikipedia to link articles under a common topic and are generally found at the bottom of the page. Clicking the category name displays a list of pages (articles) and sub-categories within it.
All of the categories form a hierarchy, although sub-categories may be a member of more than one category. An example of a small part of this hierarchical structure is shown below:
|Category:History||→ Category:History by location||→ Category:History by country|
|→ Category:History of Australia||→ Category:History of New South Wales|
|→ Category:History of the United States||→ Category:History of California|
|→ Category:History of Zambia|
The actual relationship can be a little more complex as a category may appear as a subcategory of several other categories.