Articles and categories
From Joomla! Documentation
What are articles and categories? What are they used for in Joomla! sites?
In Joomla!, an Article is a self-contained unit of written information that you want to display on your site. It normally contains some text and can contain pictures and other types of content. For many Joomla! sites, articles form the majority of the information presented.
It is important to understand that the content of the site is totally separate from the formatting of the site, the way it looks on the page. So it is best to think of Articles as pure content independent of presentation. The same Article might be shown with different fonts, colours, headings, and background, and might be shown in different locations on the page, all depending on other settings in the Joomla! CMS.
Categories in Joomla! provide an optional method for organizing your articles. A Category may contain articles and other Categories rather like a file system tree. One Article can only be in one Category. If a category is in another category it is called a subcategory of that category.
For example, you might have a Category called "Pets" and in that category a subcategory called "Dogs" and another called "Cats". Articles about dogs would be assigned to the "Dogs" Category and ones about cats to the "Cats" Category. You cannot have one Article that is in both the "Cats" and "Dogs" categories. If you have an article that is about both Cats and Dogs you could put it in the Pets category.
There are two main reasons you might want to organize your Articles in categories.
List and Blog Layouts
First, there are built-in Menu Item Types in Joomla! that take advantage of this organization: Category Blog and Category List. These Menu Item Types (also called "layouts") make it very easy to show articles that belong to selected categories. As new articles are created and assigned to categories, they will be automatically placed on these pages according to the parameters you set for each page.
For example, say you have a Category Blog layout for the "Pets" Category, and say you have it set to order articles starting with the most recent one first. When you add a new Article to the "Pets" Category, it will automatically show on the "Pets" blog page as the first Article. You don't have to do anything other than add the Article and assign it to the "Pets" Category.
Organizing Articles in List Views
A second reason to use categories is to group the articles so you can find individual articles more easily. For example, if you have 200 articles in your site, you can filter articles by Category so the list will be limited to those in the selected Category.
Categories versus Menus
It is important to understand that the structure of Categories (for example Organisms → Pets → Animals → Dogs) has nothing to do with the structure of the menus on your site. Your site could have one menu level or six menu levels.
Other Types of Categories
Categories are also used by other components, including Banners, Contacts, and News Feeds. These categories are completely different from Article categories and are set up in different Joomla! Administrator screens. So when you see something about categories, it might refer to Article categories or it might refer to categories for these other components.