Content Management Systems (CMS) are applications, or all in one processes, that allow for webpage design and content control. Wikipedia Definition
Because most of the content is stored in a database and is separate from design, users can change things like text and images without fear of altering the site design. Conversely, the design (“themes” and/or “templates”), as well as additional software (“plug-ins” or “extensions” or “add-ons”), can be imported or changed without modifying or losing any content, only changing the manner in which it is displayed.
This is different from “old” style sites that were coded from scratch or made with web authoring tools, such as Dreamweaver, Microsoft FrontPage, Microsoft Publisher, and others. These tools require someone to have full access and some ability in terms of web design, and content needs to be changed on the “back end” with access to upload the actual files that make up the web pages. Design and content and functionality may be somewhat separate (using XML, CSS, etc.) but all all managed concurrently. There are a wide variety of CMS applications, choosing one can be daunting. Wikipedia List
It is best to go with the most widely used and which fits your needs and skill level. The TOP 3 CMS today are (in order of ease of use): WordPress, Joomla, and Drupal
WordPress is the most common and “easier of the 3” , Joomla popular and has more features out of the box, and drupal which requires time, effort and expertise but your website can look anyway you wish.