File with an .MSI extension contains a compressed installation data package for Windows system. System update installers and software provided by companies other than Microsoft, within an MSI file, have information on files that are going to be installed and their final directories.

In order to differentiate system installer files from others, Windows Vista applied files with an .MSU extension, for update installers. The content of an MSI package can be unpacked with a regular file decompression tools.