A content management system (also known as a content management software, or CMS) is a way to manage site content without having to know any coding language. It has many advantages for both the novice and experienced site owner. With a CMS, a site owner does not have to rely on a webmaster every time she needs a text change.
All blogging systems are essentially content management systems, though some are more extensible than others. For example Wordpress.com, while built to be a blog system, also enables users with no web experience to create plain web sites with simple pages.
If you are working with a web designer to create a new site, asking for a built-in CMS can save you time and money in the long run. If you can find a web designer who will build on an open source (free) CMS system such as Wordpress or Drupal or Mambo (or many others!), you’ll have more options in the long run, since there are many designers and developers who know these systems.