There are various folders provided for your use that are referred to as 'subsystems' - the main one of course being the base of your website, your 'www' subsystem.
To help secure your applications, there are some extra directories 'outside' your website's root folder.
Depending on their security settings, they are useful for holding things such as configuration and database files.
By placing such files outside of the webserver's main 'www' subsystem, you avoid the possibility of them being simply downloaded if someone discovers the path.
Most visitors to your website will cause your PHP,Perl,Tcl,ASP etc code to run as an 'anonymous user' specific to your site.
Your code will be running in the context of this anonymous user and will be able to access the subsystems only as described below.
Apart from 'www' and 'mdb'; the subsystems are named to represent the permissions on the directories from the point of view of the anonymous web user - i.e the permissions a of a browsing user who has not 'logged on'.
So for example, the anonymous user has read & write permission but not delete (or execute) permission on files and folders in the 'readwrite' subsystem.
The 'www' subsystem is the only system that is mirrored from the author system over to the live webservers. It's important to remember that files under your www directory will get copied over whenever mirroring from the author system occurs.
Therefore if any of your perl scripts or ASP code etc writes to files under 'www', the info will be lost.
An exception to this is the "_private" directory, which has been set to avoid mirroring, so either get your code to write here, or to another subsystem.
You upload to the subsystems using FTP, where there are 'author' and 'live' subfolders to indicate which server is being uploaded to.
Tools such as Microsoft Interdev and Frontpage can't directly access the non 'www' subsystems.