Help Screen Description Categories
From Joomla! Documentation
Categories are are used to organize Articles, Contacts and other components for ease of management and display. They can be organised into tree-like structures like file systems. For example, category Pets may be a parent to category Dogs, category Cats and Category Goldfish. Categories can be nested to any level. All Articles must be assigned either to a parent Category or a child Category or to the special Category called 'Uncategorized'.