Content Management System
At it's core, websites are made up of 2 parts: the content (including text and images) and the layout. To edit content on flat, non-dynamic sites clients would have access to all the code of the page. Forget to close a single div tag and everything could break.
Content management systems separate content and layouts by using databases and templates. The content - editable via a backend admin site - is saved in a database. The layout - with all the html and php code - builds the pages based on the design and pulls in the content.