[dba-Tech] MS Server software and CALs

Can anyone point me to a clear, understandable read of what an OS CAL is and
why I need it?  IOW, I understand a CAL for SQL Server for each user, but
each workstation already has Windows XP Professional installed on it, the OS
is paid for, why do I need a CAL for the Server's OS?

Likewise, my client is currently at about 40 employees.  Would SBS 2003
Premium be a good choice.  As I read it, this version comes with SQL Server.
I am assuming here that the $100 / user CAL includes the rights to use SQL
Server for each user?  This appears to be less than just the CALs for SQL
Server purchased as a separate package.  What's up with that?

I'm pretty confused.

John W. Colby
www.ColbyConsulting.com