A Windows Installer Package (.msi) file is a specialized database format used exclusively for installing, updating, or removing software on Microsoft Windows systems. Unlike executable installer files (.exe) that run their own code, .msi files are interpreted and executed by the built-in Windows Installer service (msiexec.exe). This service handles the installation process reliably by managing system changes like file copying, registry edits, and shortcut creation based on the standardized instructions within the .msi file.

You typically open a .msi file by double-clicking it in Windows Explorer, which automatically triggers the Windows Installer service. System administrators often deploy .msi packages silently across networks using tools like Group Policy or Microsoft Endpoint Configuration Manager. Developers and IT professionals may also interact with .msi files directly using commands like msiexec /i filename.msi in Command Prompt or PowerShell for automation or using specialized tools (like Orca) to edit their contents.

The structured nature of .msi files ensures reliable installations, easy uninstalls, and support for repair functions. However, they require Windows and sufficient administrative privileges (triggering UAC prompts). While fundamentally tied to Windows, evolving technologies like PowerShell Core offer cross-platform scripting alternatives for simpler deployments. Their robust structure ensures continued relevance for managing complex software on Windows systems.