From Wikipedia, 2010 March 15th

A Web Content Management System (WCM, WCMS or Web CMS) is content management system (CMS) software, implemented as a Web application, for creating and managing HTML content. It is used to manage and control a large, dynamic collection of Web material (HTML documents and their associated images). A WCMS facilitates content creation, content control, editing, and essential Web maintenance functions. The software provides authoring (and other) tools designed to allow users with little knowledge of programming languages or markup languages to create and manage content with relative ease.

In short, web content management system is a system used to manage the content of a Web site. The CMS allows a content manager, who may not know HTML, to manage the creation, modification, and removal of content from a Web site without needing the expertise of a Webmaster. The CMS compiles that information to update the Web site. The features of a CMS system vary, but most include Web-based publishing, format management, revision control, indexing, search, and retrieval.