As already told before that the highest article groupings are section. You can manage the section in Section Manager. You can access it throught Content > Section Manager. Look at the picture 3.60.

Picture 3.60 Section Manager

In this Section Manager page, you can make, modify, and delete section. If you delete section which there is a category in it, the category will lose its “mother.” Because of the category must have a section, Joomla! is not allowed you to delete section which there is a category in it.