When creating new forum and setting up the different permissions (eg access, post, edit, ...etc), we have options of selecting "All Users", "Private Group" , "Registered Users", or "Admin groups"!  I don't quite understand the "private group", is that the member groups that we create?  But it does not specified which private group?

Here is a scenerio: I want to have a forum called "moderators only". Allowing only members in the moderator group to access, so which permission option should i go:

1) Setting the private group thing as mentioned above?  Or after doing that, I still have to assign that moderator to that particular forum for him to gain specific access?

2) Create a password for that forum and give all moderators the password?

3) .....

I guess I am confused with the different levels of security.  Can someone explain to me in a simplest way?