Content Management System (CMS) is a specialized software that runs at the server side. Usually it use a databases. The main purpose of all CMSes is to simplify publishing and managing site content. 
The key features of the general content management systems are: 

• Separation of design, content and structure.
• Improving the lifecycle of the site.
• The visual appearance of your web site can be changed without affecting to existing content.
• Content creation is easy and not required programming skills. Any computer user with average knowledge of word processing is able to manage the content of your site with the graphical user interface. So, authors can easily create and format text, place images or multimedia files and other content elements.
• The poweful CMSes, such as Typo3, provides a server generation of the graphics for image menu, graphical headers, badges. Therefore, the graphic design of your site is not limited to standard fonts only and you don't need to have an additional specialist to reflect text changes in graphics.
• The commonly used features like forums, polls, shopping carts, search engines, news management, generally already inplemented as pluggable extensions. Also, most CMS allows to web-programmers to create own extensions, which will provide specific functions.
• The client side usually based on a Web browser, so you can manage your content at any computer, which have internet connection, without installing additional software.
• The most CMSes has configurable access management, which allow you to grant permissions for different operations to different users. So, you can have users with such roles as authors, editors, managers, which will have limited permissions to defined by you set of operations. Typo3 has very powerful and flexible access control system.
• You don't need to hire specialists for every small changes - all content elements and other entities (depends from configuration) are managed by yourself.

In short, good CMS reduces the cost of support of your site and further development and allow you to make content changes very fast, which is extremely important, especially to represent your business in web space. 

• What is a content management system?
• Why Typo3?
• TYPO3 basics for web-developers