Question : What is SUS

HI
I am running server 2003 ent edition domain.  I would like all my clients (XP prof) and servers (server 2003)' to get microsoft patches and updates centralized position,  Is SUS a software server?.  Can it be installed on a server 2003?.  My clients should be able to still download the updates in case the SUS is unavailable.  Is it possible?
Help plz

Answer : What is SUS

Article from MS on how to perform patch management usins sus:
http://www.microsoft.com/technet/archive/security/prodtech/sus/secmod198.mspx?mfr=true

To answer your questions in order:
It's not technically a software server, it is an update service that runs on a server.
Yes, it can be installed on server 2003.
And yes, they should still be able to go to Windows Update and manually check for updates on individual workstations.

I'd recommend going with WSUS (Windows Server Update Services).  It is the replacement for SUS, and according to MS, SUS is already out of its support lifecycle:
http://support.microsoft.com/kb/905682

Here is a link to the WSUS Solution Center from MS, where you can get a wealth of information on deploying and using WSUS:
http://support.microsoft.com/?scid=ph;en-us;10844