The category tag assigns the page to a category. Pages can only be assigned to one category.
This is mainly used to group pages on the sidebar.
The categories are defined by the wiki site. End users have no control over which categories show on the sidebar.
Some tags will automatically categorize the page. For example, the function tag will categorize to whatever you set the type to.
There isn't a way to adjust the order of categories right now.
There is a dropdown selector. It'll change the page text by adding the right tag.