Content management systems provide a way to manage a website with less hassle than usual.
Most web CMS systems are primarily focused on providing templates and pre-made applications (such as polls, blogs, search). Many do not have good versioning features as you might expect given the name. Depending on what you want to achieve, you may want to look at using a Wiki instead.
http://www.cmsmatrix.org - compare and contrast different CMS
http://www.opensourcecms.com - demo sites of most opensource CMS
http://www.mamboserver.com - easy to use
http://www.joomla.org - Community fork of Mambo; has a larger community and is moving towards cleaner code
http://www.drupal.org - better code than mambo
http://www.plone.org - runs ontop of Zope
In the end I went with drupal since only the only other one that is in Debian was plone. On my machine plone required 108MB of disk, drupal only needed 18MB -- AdrianBridgett