A content management system essentially does what the name says. Its a method of efficiently and quickly updating content on your web pages. Pretty much all sizable sites these days are built on a CMS for ease of updating. Just as with other systems there are also a broad range of systems out there some more complex than others. For example although Wordpress is technically a blogging application it can be used very effectively for a small site content management. I'd recommend checking out these links below to see some of example CMS systems out there:
http://www.joomla.org/
http://drupal.org/
http://www.movabletype.org/
http://modxcms.com/
http://expressionengine.com/
Hope that helps 