Usually the final post on a thread vanishes entirely when it's deleted but a post that is deleted from the middle of a thread leaves a message stating it has been deleted.
The "invisible" users are ones that have the invisible option checked in their preferences.
The anonymous users are the ones who are looking at the forums without logging in.
“Life’s journey is not to arrive at the grave safely in a well preserved body, but rather to skid in sideways, totally worn out, shouting ‘Holy ____…! What a ride!’” ~Hunter S. Thompson