Content Management System.  A Content Management System is a stand alone application that is usually a combination of HTML, CSS, JavaScript, PHP and MySQL that allows individuals to more easily create and manage website content.  It often works with a MySQL database.  CMS systems can also be designed to work in a ASP.NET or Windows based environment as well.

The most popular CMSs are WordPress, Joomla and Drupal, but there are many other CMSs that are available.