Public can be viewed by anyone, is available via search, and will show up on the front page.
Hidden can only be accessed via the url, it will not show up in search or be on the front page.
Password Protected can only be accessed by the user that created it or via a password set by the creator. It does not show up in search or the front page.
Secret can only be accessed by the user that created it. It does not show up in search or the front page.