A Content Management System could be best described as integrating elements of a web site such as design, content, media and database into one easy to use package. The System is normally controlled by using a web interface (a form) which will capture the data and store it in a database according to its attributes (title, category keywords, description etc.). Some of the better ones out there are Mambo / Joomla, and because it's open source there is a large community are continually contributing updates and addon's to the system. Here's an example Web Site Design

The design of the site itself is controlled by templates and css style sheets making the design and development of the site is much more consistent especially larger websites. Another important factor is that because the whole site design is controlled by a couple of files it can be easily updated to give your visitors a themed season effect such as Christmas, Easter, Summer, Winter etc. This is important when it comes to repeat visitors, they know that the site is up to date and that the designer / developer is actually thinking of the visitor.

Finally, as you become use to working with a Content Manage System, you could spend more of your time concentrating on content and maybe farm out the design or purchase some templates for your site.