Articles Live in Categories.

Categories Live in Sections

But this has nothing to do with the hierachy of how people get to different sections of your site with menus, submenus or drop down menu items. If your site has < 20 pages, don't use sections or categories at all - just select "Uncategorised" for all articles. The main reason you would use sections or categories is to create blog pages (that show every article from a given category on one page - usually with a "read more" link after each intro paragraph.)