I don't see tiki_p_edit_categories myself, but I would guess that this is the same thing as tiki_p_admin_categories. Edit categories means be able to edit the actual categories themselves. It's unlikely that you'll want most users to be able to do this, as you may want to encourage them to use the category structure that you have created already (and not give them the possibility to break it).
Edit categorized means that you can edit any object that has been placed (categorized) into a particular category. Note that I say "any object", because you can categorize more than just Wiki pages.
Does this make sense?