Categories
Categories were introduced to MediaWiki in version 1.3.0 which is not currently running here.
You can see all current categories here
Categories are basically automatic two-way hyperlinks where when you add a page to a category by adding [[Category:SomeCategory]] to the bottom of the page a link is created to the category from the article and to the article from the category, thus it is a two-way hyperlink.
Categories may be nested ie. they may contain sub-categories.
Preliminary plans are to use categories in Research Wiki pilot for at least Product Groups where product groups are represented as Categories (to enable grouping of products and product substitution suggestions) and Campaigns (to determine that one doesn't accidentally vote for more then one campaign in each campaign category). Products will be tied to a Product Group category as precise as possible.
Other uses may rise.
For more reading on this proceed to the Meta-Wikipedias user guide to Categories