In Joomla! a Category is a collection of Articles. In Joomla! versions up to and including , is the middle level in the hierarchy Sections -> Categories -> Articles. For example, a website might have Sections called "Animals" and "Plants". Within the "Animals" Section, the website might have Categories such as "Birds" and "Mammals". Within the "Birds" Category, the website might have Articles such as "Parrots" and "Sparrows". In Joomla! versions and up, Categories can contain other Categories. The example above could be created using an "Animal" category, the Categories "Birds" and "Mammals" that are in that category, and the "Parrots" and "Sparrows" Articles in the "Birds" category.
Categories are maintained using the Category Manager (help screen) which can be reached in the Administrator (Back-end) by clicking on the Content menu, then the Category Manager menu item.