From the MSDN documentation for the MetaWeblog API about the getCategories() method:

Returns an array of structs that contains one struct for each category. Each category struct contains both a description and title field which each contain the name of the category.

From the XML-RPC documentation for the MetaWeblog API about the getCategories() method:

The struct returned contains one struct for each category, containing the following elements: description, htmlUrl and rssUrl.

Somebody just stab my in the eyeballs, please.