$wgTrackingCategories is the list of tracking categories which are automatically added to pages. Starting with MediaWiki version 1.25, this configuration variable is deprecated and the default value changed! The default tracking categories are now hard-coded in the MediaWiki code and Extensions should now register tracking categories using the new extension registration system. They are listed on Special:TrackingCategories.
People who write extensions that add tracking categories via $parser->addTrackingCategory should add the system message corresponding to their tracking category to this array. They should also define a message with the same name but -desc appended that describes the criteria for a page to be tagged with the tracking category.