CMS stands for Content Management System. In laymen terms a CMS can help you build a web presence and is highly customizable. A CMS is a web portal software that gives you the ability to edit and update content on the fly. With built in functionality for your users to join your site, it is a coveted asset for anyone hoping to build a successful website.
Many of the CMS software packages out there are free, and all of them have "plug ins" that stretch the potential of your site. Some of examples of CMS systems are:
Wordpress - This actually started as a blogging package but with a small learning curve and wonderful templates it has become one of the leading CMS systems out there. Chosen by many for ease of use, and flexibility.
There are many more packages out there. However these are the key three. A CMS has the ability to create static pages, and the ability to post to dynamic pages on the fly. With this ability you can hire a designer to create you a professional template but can edit your own content afterwords.
The amount of features are jam packed and we could spend all day discussing them. Trust me if you want to build an easy website, a content management system is the wy to go.