At JAM, every website we build uses our custom-built content management system. A CMS is the application that allows for publishing, modifying, organizing and deleting content on a website.
To access the CMS, a user types in the URL of their website, followed by a specific subdirectory (like "/admin-login") for example.
From there, the user enters their username and password and is granted access into the "back-end" of the site. Once logged in, the user can make whatever modifications they need to make.
Every website has a CMS. The three most common are Wordpress, Joomla, and Drupal. Together, these three account for more than 70% of the CMS market.