Semantic MediaWiki is an extension to MediaWiki that essentially allows a user to tag certain information as metadata for the page. But then you can define how that metadata relates to other page metadata. It's kind of like a category tag, but much more structured. It's almost like being able to define ''HOW'' or ''WHAT KIND'' of link exists between a page and the information on the page. Of course, the big pay-off is being able to sort and sift by that information using a simple query. When used together with the parser functions extensions, it's very, very powerful.