A Content Management System (CMS) is software that allows the owner of a website to edit and add content ( text, images, video etc ) usually through a web browser interface. CMS take many forms but they all remove or reduce the need to have a developer make changes to website, saving time & money and putting the site owner in control. Usually pages are template based and content is inserted into specific areas of a page including sidebars, navigation, headers etc.
There are many different types of CMS ranging from entry-level for small businesses to enterprise-level and beyond. WordPress, while technically not a fully-fledged CMS, is one of the most popular pieces of software to be used as as a content management system with over 6million sites running on it. Other popular CMS include Drupal, Joomla, Concrete5 and, at the other end of the spectrum, software like Microsoft Sharepoint & Adobe CQ5 (previously Day CQ5).