Drupal (CMS) - https://en.wikipedia.org/wiki/Drupal
It is used as a back-end framework for at least 2.1% of all Web sites worldwide ranging from personal blogs to corporate, political, and government sites. It is also used for knowledge management and business collaboration. Drupal core, contains basic features common to content management systems. These include user account registration and maintenance, menu management, RSS feeds, taxonomy, page layout customization, and system administration. The Drupal core installation can serve as a simple Web site, a single- or multi-user blog, an Internet forum, or a community Web site providing for user-generated content.
Drupal Modules - https://www.drupal.org/project/project_module
Extend and customize Drupal functionality with contributed modules. If a module doesn't quite do what you want it to do, if you find a bug or have a suggestion, then join forces and help the module maintainer. Or, share your own by starting a new module.
CMS - https://en.wikipedia.org/wiki/Content_management_system
A content management system (CMS) is a computer application that allows publishing, editing and modifying content, organizing, deleting as well as maintenance from a central interface. Such systems of content management provide procedures to manage workflow in a collaborative environment. These procedures can be manual steps or an automated cascade. CMSs have been available since the late 1990s. CMSs are often used to run websites containing blogs, news, and shopping. Many corporate and marketing websites use CMSs. CMSs typically aim to avoid the need for hand coding, but may help it for specific elements or entire pages.