A website Content Management System (CMS) is an application that allows you to edit, update and configure your website via a web based control interface.
The basic structure of these CMS applications is two fold. First, a database is used to store all the website's content, information structures, media references, etc. Then second, a middleware technology is used to present an interface between a website manager or 'editor' and the database. It's this middleware interface that allows for maintenance of your website without requiring indepth knowledge of web technologies.
At a basic level, a common use of CMS's is to produce blogging engines which allow you to post articles with others providing feedback by adding comments. But these same CMS's are also used to power web applications such as Facebook, MySpace or YouTube.