What exactly does "backend" mean?

[Jim C <jcllings@tsunamicomm.net>]

OK, so I've configured an OpenLDAP server and 3 clients.  
I've seen references to useing different "backends".  
This would seem to imply that one could first install an LDAP server and 
use, for example, MySQL as a backend
then, fire up MySQL and do SQL based searches on the database.  If this 
is correct, it would be most useful.

Also can anyone point me at a good tutorial for authenticateing Win2K 
and WinXP clients off of OpenLDAP?
Can this even be done?  My research to date suggests that it cannot.  

Toss me a bone here. ;)