A content management system is a computer application that allows publishing, editing and modifying content, organizing, deleting as well as maintenance from a central interface.
In a simple term you do not have to know html, css and php. Once a Web Developer develop the website and give you, you can do your own editing. Web Master can appoint different users with different authority. You do not have to depend on somebody from outside, you can edit your own website everyday, Ideal for newsletter, newspapers and restaurants.
Also most of CMS packages are free and open source.
Most widely used cms packages are Joomla, Wordpress and Drupal.