What is a Content Management System (CMS)?
A Content Management System (CMS) quite literally allows you to control and manage the content within your web site - without technical training. Using this uncomplicated system you can easily add, delete images and edit text in your web site on the fly. You can also have an unlimited number of pages and a full site-search engine.
The definition of a CMS is an application (more likely web-based), that provides capabilities for multiple users with different permission levels to manage (all or a section of) content, data or information of a website project, or internet / intranet application.
The main purpose of a Content Management System (relating to web) is to provide the capability for multiple users with different permission levels to manage a website or a section of the content.
For example, you can take a website which has Articles, Blogs, Press Releases, Store, Events and assign each section or a part of a section to user(s) to create, edit, and archive.