Let's strip away the mystery behind this acronym and get to the bottom of what CMS really is!
Traditionally websites were created out of nothing and placed on the internet by individuals (or organisations who employed individuals) with little or no social skills or desire to be placed in direct sunlight. Now I can say this because there is still a part of me (a very small part I might add) which descends from these types of individuals.
These people (let's call them developers) are the very best at taking lovely designed websites and ensuring they appear on the internet in the most efficent way. Problem is when the client (who does not speak the same language as the developers) wants to change something on their site.
What the client might see as a simple change of an image or special offer can in fact be a couple of hours of work for the developer - and us developers don't come cheap, despite living in basements/cupboards with nothing more than a laptop and a lunchbox prepared by mummy.
So along come tools which allow websites to be served up in a way we can easily add pages/change content without having to get the developer involved. These tools have now become systems and there are a shed load of them out there. the more famous ones being Wordpress, Joomla or say Drupal.