Bah, slacker!
generic_wrapper is the name of the sub in public_common that's responsible for the generation of most of the top of the page. "Top of the page" generally means just the actual visible HTML, but it can also set the page title.
Most generic_wrapper calls are in templates. For example, the one for the forum summary is in public_forum_summary. Real shocker there.
page_name and page_title?
page_title is only the title tag.
page_name, if set, is BOTH the title tag AND the name of the page in the breadcrumbs (if no other breadcrumb is specified). The two are often the same... but not always.
Compare the generic_wrapper call on line 20 of public_forum_page with the generic_wrapper call on line 10 of public_forum_summary for varying arguments, etc.