A Content Management System (CMS) is used to manage a websites content. It provides a template that you can add information to on a regular basis without having to download a file from your website, edit it in notepad and re-upload to the webserver. Access the CMS online and edit content when you need to.
A CMS will support the management and publishing of your website, cover the lifecycle of the content on your site, manage the structure and navigation of your site and also the appearance of your sites webpages.
The article at Step Two Design provides a great overview of a CMS system and how it will play an important part of your online presence.