Radiant is an open-source content management system (CMS) designed for building and managing web applications and websites. It is built on the Ruby on Rails framework, which provides it with flexibility and robustness for developers. Radiant offers a simple and elegant interface, making it user-friendly for content editors. Some key features include custom page templates, an extensible architecture through its rich library of extensions, and a powerful templating language for dynamic content. Radiant is particularly suited for web developers who are comfortable with Ruby on Rails and seek a flexible, lightweight CMS solution.