Windows Software Update Services (WSUS) is Microsoft’s implementation of a local centralized server which is set up to collect, allow for approval, distribute, and report on updates and patches normally downloaded directly from Microsoft.
Instead of clients going directly to the Microsoft update service (through Windows Update), clients point to the local WSUS server located on the UD network.
Client side targeting allows the client to specify which WSUS groups they belong in for approval and reporting.
The updates are approved centrally by a WSUS algorithm which decides which updates are approved for which groups of computers and when the updates will run.
Once updates are approved, the client computers can be set up to install them on a schedule automatically, to download and notify, or to only check for updates when a local administrator checks using the Windows Update control panel applet.
Regardless of whether a local system participates in WSUS, the local client computer administrator still retains the ability to check for updates directly from Microsoft at any time.