Windows Update Services (WUS) is the new name for Software Update Services (SUS). Except it might not be. Last week, Thomas Lee (who is well placed to comment on such things, being both a Microsoft Regional Director and an MVP) quite rightly pointed out that WUS a) sounds bad; and b) is not an accurate description of what the product does.
For more information about SUS/WUS see SUSserver.com and the Windows Update Services Wiki.