In its simplest form, a content management system (CMS) is a database driven software application that lives on the web and it pulls data (information) from the database and presents it consistently to a web visitor’s browser through a template. The template usually consists of design and layout features such as a standard header, footer, primary navigation bar, columns, rows, background patterns, background colors, font type specifications and other attributes that add style and consistency to a web page.
In WordPress, a commonly used open source CMS, these templates are called Themes. Basic functionality such as reading posts, writing posts, navigating the site, searching the site, delivering basic information is performed by the CMS itself. However, the are often additional features and functionality such as managing photo galleries, calendars, e-commerce, etc. that are performed by special applications that work with or in conjunction with the CMS. In WordPress, these applications are called Plug-ins. In Drupal, they are called Modules.
Once the CMS is setup, designed, and stable; it mostly runs on its own. No maintenance required, no moving parts. With CMS solutions like WordPress, little or no technical skills are required to manage web content. Drupal is a bit more technical and complex, but for managing simple things like news, blog posts and pages, marketers, communications officers, and public affairs specialists can manage the web content without technicals skills, experience, or assistance from an IT department.