Somewhere between CMS and Scaffolding Framework
MODx is a PHP based, mm.. tool, for publishing websites. I don't want to call it a CMS, though If you read MODx documentation, they'll call it a Content Management System. But in my experience products labeled CMS tend to be inflexible tools that require lots of effort to place widgets on custom templates. I remember using one tool in which you could not have a publicly accessible page not belong to a menu. We ended up building a hidden menu for all such pages - absurd!
At the other end of the spectrum are highly flexible frameworks. These are much more flexible, and let you run wild, but leave a good chunk of work to setup decent content management, authorization, etc. before you even get to the site at hand.
Well MODx sit nicely...