A Windows Installer patch (.msp file) is a file used to deliver updates to Windows Installer applications.
A Windows Installer patch is a self-contained package that contains all the information required to update the target application. A patch package (.msp file) can be much smaller than the Windows Installer package (.msi file) for the entire updated application. A Windows Installer patch is merely a packaging mechanism for an existing MSI update type such as minor or major upgrades. A working minor or major upgrade is required for a patch to be possible to create.