What is a Content Management System?
A content management system is a system used to manage the content of a website. It should provide an easy-to-use interface for creating, managing and updating content on a website, Intranet or extranet. For content contributors, making changes to a page should be as easy and straightforward as using Word. For publishers, it should provide all necessary tools and workflow to administer content production with in accordance with business rules.
To sum up
"A CMS is a tool that enables a variety of (centralised) technical and (de-centralised) non technical staff to create, edit, manage and finally publish (in a number of formats) a variety of content (such as text, graphics, video, documents etc), whilst being constrained by a centralised set of rules, process and workflows that ensure coherent, validated electronic content."