Windows Installer XML (WiX) is a toolset that allows developers to create installation packages for Windows applications. It uses XML files to define the structure and behavior of the installation process, making it easier to manage complex installations. WiX is widely used because it integrates well with other development tools and supports various features like custom actions, user interface customization, and localization. By using WiX, developers can ensure their applications are installed correctly and efficiently on users' systems.