All content in Joomla is organized by categorization.  Think of a category as a filing cabinet for your content.  You must define your categories before beginning to add your site content (called articles). Your site plan will help you organize how your content will fit into categories.  By grouping similar articles into categories, you can easily manipulate different types of menus/modules to display dynamic content.  Dynamic content also allows users that may be less technical to add content without having to alter the navigation structure of a site.  Categories are the foundation for building our navigation structure and organizing our content (articles).   

When you first approach Joomla, one of the most important things to understand is how the system itself is organized.  In order to help you remember what to do, think of this acronym – C.A.M.

C – First, you need a Category
A – then create your Article and assign it to the appropriate category
M– then create a Menu item linking to the article and enable the module

The first step toward adding new articles or building new navigation is to start with adding a category.  The category allows us to organize our articles by similar content.  Categories are used when building menus that display dynamic content, such as category blogs, featured blogs, or category lists.