From Joomla! Documentation
In Joomla! an Article is a piece of content consisting of text (HTML), possibly with links to other resources (for example, images). Articles are the basic units of information in the content system and the bottom level in the content hierarchy. Since Joomla! , each Article is in exactly one Category. A Category can be in another Category making it a sub Category. It is also possible to have Uncategorised Articles. These articles exist without being associated with any Category.