Joomla organizes web content by a category hierarchy. Categories are like outlines, giving structure and organization to web content.
Each article is associated with a single category or sub-category. Menu elements then leverage categories to easily create lists of articles, blogs, and more. This tutorial describes how to create categories and sub-categories. 

Prerequisites

Administrator access to a Joomla site (version 5 or above). See 1.0 Admin Login.

Creating a category and sub-category

Steps

1. Login to Joomla as an administrator and navigate to the Home Dashboard.
2. In the left side menu, expand Content and click on Categories.
3. Click on the New button. A new-category form appears.
4. Enter a title then Save & Close. The categories list returns with your new category added.
5. Add a sub-category. Click on the New button again.
6. Enter a title, and select a parent category in the Parent field, then Save & Close. The categories list returns with your new category added as a sub-category.

Concepts 

• Articles are organized by categories in Joomla, giving structure to your website, making them easier to find and assisting in navigation. 
• Sub-categories may have sub-categories.