What is CMS?
CMS (Content Management System) is a control and operation system of the web site. The program is specifically written for use on the Internet-servers.
Control systems are used for simplification of the management of your web-site; allowing editing pages of your site in a convenient and habitual type (for example in the environment similar to text editor Word from Microsoft). CMS does not require any special knowledge. By development of your site in CMS all necessary functions are integrated and laid out. You are only required to know the content of your personal site, and to type the text or to duplicate it from other sources.
In this day and age most serious projects do not operate without similar management system in place.
Earlier when sites did not use CMS, they had been static HTML sites. To change contents of any page, it was necessary for you by means of FTP-access to a server to download HTML file of a specific page, to open it in the special visual editor and to edit, then again to fill in a file on a server with that having updated the page. If you needed to change design or a pattern of each page you needed to repeat same operation with each and every page.
Now if you have a CMS you can reach the administration panel of your website by simply logging in to a special address of the administration panel. From this panel you will have complete or partial access (depending on specified access rights you grant to each of your employees) to the website controls; templates, texts etc. The specifics will depend on the CMS, and specifications put in place by the webmaster creating it.