The weblog-portlet parameters "tag" and "ptag" are filtering options that can be used to show in a weblog listing (chronological listing of wiki pages) only pages that have were tagged by the current user with the provided "tag", or which were tagged by arbitrary users with the given tag ("ptag"; abbreviation for popular tag). These parameters are used e.g. on http://openacs.org/xowiki
when someone clicks on a tag in "Your Tags" or "Popular Tags" in the left hand menu.
The includelet "my-tags" lists for the current page the tags that were attached by the current user to it (... and it allows to add new tags, edit existing ones, etc.).