You probably wonder from where such an odd name came? Once upon a time in America there was an operating system known as Unix. It had a user with unlimited access privileges. This user was referred to as the Super-User. Super-User had a user identification (uid) of zero. I found this to be a very novel idea and began creating administrative accounts for myself on Windows systems called SU. Eventually I decided to add the first initial of my first name. This became a kind of scheme for naming administrative accounts on Windows. Using this scheme, when you want to know who your Windows administrators are, you just look for users whose accounts begin with "su".
If you would like more information on the Super-User You can obtain more information at the KernelThread web site.
Unix is still out there and if you're interested you can follow-up on one of these sites:
SUD Town where interesting things can always be found!