Manual:Categories

Categories in MediaWiki are used to group similar pages, and groups of similar groups, together to form one or more treelike 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.

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

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.