Microsoft Installer, an installer system written by Microsoft for Windows platforms.MSI files are basically relational database files that contain all the information that the Windows Installer requires to install or uninstall an application and to run the setup user interface. Installation packages are organized like the diagram above. Each package has one or more independent features. Each feature can have one or more components. The installer always installs or removes a component from a user's computer as a coherent piece. A component can be a single file, or a group of files, a registry setting, COM objects, resources, etc.