Files with the .MSI extension are formerly "Microsoft Installer" and are now "Windows Installer" Package files. It is designed for installing software on computers running modern versions of the Windows operating system. The Windows Installer system is also used to remove currently installed software and replace or repair preinstalled software.

An MSI file contains all the necessary information to install the software, including the files that need to be installed and where to install those files on the computer.

The MSI file format stores installation information in an installation package, usually with the files to be installed themselves. These packages are relational databases. They are collections of data placed in formally organized data tables, as opposed to hierarchical or network model databases, where data must be accessed sequentially. This means that Windows Installer has faster and easier access to this data than other types of databases.