The WUA scans client computers to determine what security updates have not been installed and then searches for and downloads the updates from Microsoft's update websites or from the Windows Server Update Services (WSUS) server.
According to Microsoft, there are five parts to the WUA:
Service Availability - detects, downloads and installs Windows updates and updates for other programs.
Server Connectivity - an application program interface (API) that allows client computers to connect to the WSUS server or Microsoft's update websites.
Update Detection - checks for and installs updates daily.
Update Downloads - downloads available updates from WSUS, Microsoft Update or Windows Update.
Update Installation - installs available updates from WSUS, Microsoft Update or Windows Update.