The Full Form of WSUS is Windows Server Update Services.

Windows Server Update Services (WSUS) enables information technology administrators to deploy the latest Microsoft product updates. You can use WSUS to fully manage the distribution of updates that are released through Microsoft Update to computers on your network. This topic provides an overview of this server role and more information about how to deploy and maintain WSUS.

A WSUS server provides features that you can use to manage and distribute updates through a management console. A WSUS server can also be the update source for other WSUS servers within the organization. The WSUS server that acts as an update source is called an upstream server. In a WSUS implementation, at least one WSUS server on your network must be able to connect to Microsoft Update to get available update information. As an administrator, you can determine – based on network security and configuration – how many other WSUS servers connect directly to Microsoft Update.