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 [http://svn.wikimedia.org/doc/classCategoryPage.html CategoryPage] and [http://svn.wikimedia.org/doc/classImagePage.html ImagePage] objects are child objects of the Article object, and are used specifically for Category pages and Image pages.

Replacement
Use the Context object to get what you need. Since 1.19, Context objects have a  to access the corresponding   object. Where you get this context object from depends on where your code is running. Many major classes extend, which means you can just do   to get the context and   for the   object. As of this writing that includes,  ,  ,  ,  ,  ,  ,  ,  ,  , and   . Some other classes support  which don't have   as a parent class, such as   (So if you're writing a , you can often do   to get the context).

If you need to use a method which is in  but not   and have a context object, say , you can use  , please also note the following things:
 * The displayed revision's ID is accessible through  or   and the fact that this is the current revision of the page can be accessed through   (instead of  )
 * The text of the latest revision of the current page (not necessarily of the displayed revision) is accessible through ; depending on what you use   for, you may prefer this one.