Tag page
A tag page is a page which describes a tag (i.e. it is categorized in the Tag Category). When you make a normal wiki link to the tag page, the presence of the link marks the page you are adding the tag to as containing the tag. This allows the page to be found and listed from the tag page by using the What links here link from the Toolbox on the tag page.
For example: The Stub page page is a tag page, because it is used to locate 'stub' pages around the wiki, which should be fleshed out. Any new page that you don't have time to put in a full description of, you can just say: This is a stub page for such and such. And later on, you or someone else can find stub pages to flesh out by going to the Stub page tag page and clicking on What links here to get a list of all the 'stub' pages in the wiki.