Help:Category
A category groups together various related articles. The category page itself is a page where all articles with that similar theme are listed. An example is Category:Stubs. All articles that contain [[Category:Stub]] formatting will be in this category (notice that {{stub}} automatically includes that category).
To directly add a page to a category, use the [[Category:Insert Category]] code. This page, for example, displays the [[Category:AoPSWiki]] code at the bottom.
To fix the alphabetization scheme on the category page, you can pipe a category. For example, [[Category:Mathematicians|Descartes, Rene]] will place Rene Descartes under D, not R.
List of Categories
Here is the official list of categories.
See also
This article is a tutorial about the AoPSWiki or AoPS Forum.