Web Content Management rather like the word multimedia can describe a multitude of applications and techniques that allows users to manipulate data or the 'content' of their website. The application can be;
On the other hand a Web Content Management System or CMS has come to mean ...
A content management system (or CMS) is a system used to organise and facilitate collaborative content creation. Recently, the term has been associated with programs for managing the content of web sites. Web content management (WCM) is also used to refer to these programs 
... a little bit of a mouthful. Basically you get to use a tool that allows you to change your site online, all of which is really rather useful for three very good reasons...
-
You get to own your site: no more waiting for your web agency to do your changes
-
You get to change your own site: if you can use a word processor you can change your site and keep it up to the minute
-
Your site is always up to date: not only the site's content through your efforts but also in nice online technology terms as the system 'enables' you to do more easier
Typical content management scenarios would be;
The above scenarios are often achieved using individual online applications and databases written specifically for the site and its owner. A good CMS will handle all the above scenarios and more.
If you would like to know how then simply click on each description above to find out.