A category is an admin level tag. This means that only users with administrative rights or write access can assign articles to categories while readers can not.

Categories make it possible to group articles in different sections in one location. Unlike sections, an article can be assigned to more then one category. This allows you to filter, group and even search content in a section.

You can either do this when you first create the article or at a later time by editing the options for an article.

In the "Publishing Options" section, go to the "Categories" header. Check the checkboxes for the categories you want to assign to the article.

An article can only belong to one section. But, you can make an article appear in a parent section by changing the "Display Order" for the section to either "Newest_first" or "Newest Per Section". The "Newest_first" option will cause the latest articles published in both the section and subsection to display on the section page. The "Newest Per Section" option displays the newest articles in each sub-section in the parent section page. Please see How do I change the Content Display Order for a Section? for more information.