Permit me to elaborate. So, you are in Drupal or Plone or Sharepoint and you are looking at a page. Maybe the page has content which is rich text formatted. Maybe the page also includes a link to a MS DOC file or PDF. The content of this page is the document that you are collaborating on. If so configured, then you will also find some kind of "reply" or "comment" link at the bottom of that page. You also see everyone else's previous comments about this document. This is how team members can collaborate on that document instead of emailing each other about it.