A Category is a special term used within the wiki engine to which will create a link of subsequent pages within that category. It is highly recommended that every page or category be a member of another category. A category can be referenced by a link to "Category:Category Name". If you place an internal link in your page to this, your page will be added to that category and a pre-formatted link will be placed at the bottom of the page. If you need to link to a category without adding the page to it, simply put an extra colon in front of the link - ":Category:Category Name".
A list of categories can be found at Special:Categories and it is required that you check that list before creating a new category, to make sure a relevant category does not already exist where your page can be stored.
A page can belong to more than one category. Simply place two or more relevant internal links to the applicable categories.