All CMS-driven websites rely on one form of a database or another. This database contains almost all of the content and the configuration of the site. For some sites, almost everything except the aesthetics of the site is stored in the database.
A database differs from regular, boring file storage in that it stores data in a structured, organised manner. This organisation means that one piece of information can relate to another and be organised, and then searched, in a structural manner – not unlike browsing the shelves in a store. These search requests are known as queries. You can do a lot of cool stuff with a simple database, and a smartly designed DB will carry a site a long way.