A content management system is just a fancy way of saying.  A web site with complex functionality where the user doesn't have to be a web designer to make changes to the content of the site.

The editor sees the web site represented in terms of pages and the content within those pages.  There is no need for the editor to understand HTML or any other low level details of how the web page gets displayed.  The editor simply edits the text and presses save.  The content is updated on the website instantaneously.

The benefits of a CMS system:

• Allows even inexperienced editors to safely edit content on the web site without the need of hiring a web designer.
• Reduces the amount of overhead in making small changes to the website such as adding new pages or changing content.
• Doesn't require any special software like Frontpage or Dreamweaver, CMS run directly from your browser.  You edit your live website.

If your company needs to make frequent updates to the content of your website, but you have to rely on a web designer to do all changes then CMS is a good solution for you.