Separation of layout and content
The content is kept isolated in a database (structured data) or in a file-system (unstructured data).
The administration of the content takes place in the database. The content is integrated into the templates, which contain marks intended for this at the relevant places, when it is published.
In addition to these content-tags there are plenty of control structures and mechanisms that influence the appearance of the website or a print product (e.g. automatically created indexes of contents or showing/omission of specific code-blocks).