Plugins are the means with which you can greatly enlarge the functionality of your CMSimple_XH website. Even some parts of the standard CMSimple_XH are programmed as plugins, meaning, they can easily be replaced by different plugins. These are the pagemanager, the filemanager and the editor.
Popular plugins provide picture galleries, guest books, blogs, comment possibilities, integration of video, calendars, etc.
Besides, with a little knowledge of php you may quickly discover how easy it is to write your own plugins.