It is a service designed to allow software to be distributed whenever you get an update. It is used in conjunction with system restore because typically when you receive an auto-update from MS, system restore launches in case something goes wrong with the update. YOu can then roll it back to a "pre-install period".
http://www.microsoft.com/smallbusine..._overview.mspx
That site there explains SUS 2.0 which is basically (AFAIK) the same as SDS 2.0 apart from the fact that SDS is auto-installed and SUS 2.0 is used by system administrators to control what updates from Microsoft get installed on their LANs.
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