A Content Management System (CMS) is an application through which you can easily create and manage dynamic web content.
A CMS separates the different parts of a website (content, design, functionality) and thus allows for easier website management. When using a CMS application, you can easily divide the responsibilities of managing different parts of your website to different people. For example, non-technical people can be responsible for the site's content, while more technical people can work on the site's design and functionality.
Most CMS applications have built-in tools which will take care of most tasks for you. This way changing your site's design or adding a blog, for example, can be done in a matter of minutes.
And the best of all, you don't need any programming skills in order to use a CMS and create a gorgeous website.