A Content Management System (CMS) is a software system used for content management. Content management systems are deployed primarily for interactive use by a potentially large number of contributors. For example, the software for the website Wikipedia is based on a wiki, which is a particular type of content management system. For the purposes of this page, Content Management means Web Content Management.
Part of the definition from WhatIs.com:
A content management system (CMS) is a system used to manage the content of a Web site.
Typically, a CMS consists of two elements: the content management application (CMA) and the content delivery application (CDA).
The CMA element allows the content manager or author, 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 CDA element uses and compiles the information the author puts in to update the Web site.
The features of a CMS system vary, but most include Web-based publishing, format management, revision control, information indexing, search, and retrieval.