File with a .MSU extension contains a data pack for Vista Microsoft Windows system – metadata describing a package, a properties file used during the installation process, CAB files containing an update, and an XML file informing about the content of a package.

MSU files are used automatically by Windows Update Stand-alone Installer – Wusa.Exe, which is an element of Windows update service. Previously, this was a function of the MSI file. The content of files can be unpacked and installed manually with Windows command line.