Windows Installer is a system service for installing and managing applications. It is considered as an engine which can triger, control and manage software (also widely called as application packages) in a computer. It provides a standard method for developing, customizing, installing, and updating applications.

Windows Installer is an operating system service that allows the operating system to manage the installation process. The technology is divided into two parts that work in combination: a client-side installer service (Msiexec.exe) and a package file (.msi file). Windows Installer uses the information contained within a package file to install the application.

.MSI Package is often used in exchange to Windows Installer. Extension .MSI originally stands for Microsoft Installer and later heavily referrenced as Windows Installer.