In Joomla! a Category is a collection of Articles. In Joomla! a website might have Categories called "Animals" and "Plants". Within the "Animals" Category, the website might have Sub Categories such as "Birds" and "Mammals". Within the "Birds" Category, the website might have Articles such as "Parrots" and "Sparrows".
The example above could be expanded even more with specific articles on different species of Parrots and Sparrows. Start with using an "Animal" Top Category. Placing the Sub Categories "Birds" and "Mammals" are under the Animal category, and then a "Parrots" and "Sparrows" Sub Category under the "Birds" Sub Category as shown.
Now you can create multiple articles in the Parrot and Sparrow Categories.
Categories are maintained using the Category Manager which can be reached in the Administrator (Back-end) by clicking on the Content menu, then the Category Manager menu item.