WiX is a free and open-source toolset used for creating installation packages for Windows applications. It allows developers to define the installation process using XML files, which can be compiled into a standard MSI (Microsoft Installer) format. This makes it easier to manage software installations, updates, and uninstalls. The WiX Toolset includes various components, such as WiX Toolset Visual Studio Extension, which integrates with Visual Studio to streamline the development process. It is widely used in the software industry for its flexibility and ability to create complex installation scenarios, making it a popular choice among developers.