Manual:Categories

Categories in MediaWiki are used to group similar pages, and groups of similar groups, together to form one or more tree-like hierarchies.

Database
MediaWiki keeps category data in the category and categorylinks tables.

API
Several API modules provide access to category information: API:Categories, API:Categoryinfo, API:Allcategories, and API:Categorymembers.

User interface

 * Special:Categories lists all categories which have at least one page.
 * Special:RandomInCategory

A number of category extensions are available; one of the more popular ones, CategoryTree, provides a dynamic view of the wiki's category structure as a tree.