Ed,
A Content Management System (CMS) attempts to automate many of the time consuming chores associated with running a successful website. Most prominent, is the ability to add content/pages to the site WITHOUT have to touch the code. Adding content can be less trivial than even filling out a form to purchase something online. Once the content is saved, the CMS will automatically generate the necessary links for accessing that new content. In addition to that, the CMS can be loaded up with "plug-ins" that extend this base functionality to do a number of important tasks that make the dissemination of your generated content almost effortless. Some of these tasks include:

- Optimizing your content to be placed higher in search engine results. This is called Search Engine Optimization. (SEO)
- RSS feed. This will usually contain the titles of your most recently generated content and are available for syndication to anyone who wants to act as a proxy for information coming from your site. The CMS will update this automatically, thus all consumers of your RSS feed(s) will be automatically updated.
- Submission to social networks: Such as Facebook, LinkedIn, Twitter and sites that keep track of social networks like Digg and Technorati.

There are so many to list, in fact, as I write this, for WordPress (a CMS) alone, there are 11,082 plugins, 112,962,428 total downloads, and counting. Anything from banner management to anti-spam functionality, and the best part is: it's FREE!

The plugins/downloads available on the WordPress, Drupal, and Joomla! (all CMS) websites are free of charge and plug right into the CMS you have chosen, though each plugin is only compatible with its respective CMS and not the others. All you really have to worry about is downloading a plugin that is for your version of CMS as they have developed exponentially over the last few years.

So to recap, the aim of a CMS is to keep you out of the code and to keep you and your business seamlessly generating online content and tailoring that content to be store and shared in a way that matches your business needs/desires.

I hope that helps!