Categories can be layered within one another to create many, easily digestible, subsections for your site. A category page displays category and guide subpages within the detailed category box. Even if other types of articles exist underneath the category page, they will be hidden from the detailed view. Only the direct subpages are displayed, even if other category and guide pages exist lower down in the hierarchy.
Guide pages are a great location for lower-level articles, i.e. topic, how-to, and reference articles. A guide subpage in a detailed category displays these lower-level articles if they are direct children of the guide. The guide also displays any non-classified articles.
If you don't want your categories to display any subpages, choose the simple category view.