A Content Management System (CMS) is an application that provides the means to edit and publish content on a website. A CMS can abstract the code from the site so that users can focus on creating content rather than coding the website. While there are stand-alone CMS applications, many websites use one of the many free open-source CMS's available. The most popular CMSs are WordPress, Joomla, Drupal, and Sharepoint.
WordPress is by far the most popular CMS in use today. It's free, it's relatively easy to use, and there are tons of themes and plugins available to make creating pages or publishing blog posts simple. It allows users with even very limited technical skills to set up a blog in minutes.