Now you are talking web development. CMS stands for Content Management System and, as its name implies, it's all about content. Simply put, these sites are driven by a database (where all information is stored) that outputs content within a design, called "template".
For example, you can use a CMS to power static websites, blogs, e-commerce stores, forums social networks, online courses, membership sites, portfolios, etc.
The most used CMS are Joomla! (our favourite), WordPress, Drupal.