Content  is organized using categories through a system called "taxonomy". A taxonomy has different "terms" that are used as categories for articles. When you're adding an article, you might find a drop-down list of topics. By selecting one, you choose where on the site to categorize your article. If this seems hard to relate to, you can think of topics as being like folders on your hard drive--they help to organize content, so that you can find similar things in the same place.