Categories will help you to classify your questions and to divided them into different topics. When creating a question you can specify categories to be assigned to the question. You can use also the categories to search specific questions via . A category is actually a folder and has its functions. There are created shortcuts to the questions in the categories. You can add subfolders to a main category (category root), thus you'll have more categories. The category root is represented as a tab in the interface and the subfolders as subcategories:

Please, note that the category root folders are determined by your pAdmin.