Manual:$wgArticle

Details
The Article object encapsulates access to the "page" table of the database. The object represents an article, and maintains state such as text (in Wikitext format), flags, etc.

The CategoryPage and ImagePage objects are child objects of the Article object, and are used specifically for Category pages and Image pages.

Deprecation
$wgArticle has been completely deprecated in 1.19, so you should not use it anymore. Ever.