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In Joomla! an Article is a piece of content usually consisting mainly of text, but may contain other resources too (for example, images). An Article is most often regarded as the third 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". In the "Birds" Category there might be Articles called "Parrots" and "Sparrows" which describe the relevant birds in detail. However, it is possible to have Uncategorised Articles that exist without being associated with any Section or Category.
Articles are maintained using the Article Manager (help screen) which can be reached in the Administrator (Back-end) by clicking on the Content menu, then the Article Manager menu item.