Content Management System (CMS) is a much-used and confusing phrase. It means a system that allows you to maintain the content on your website. Depending on the content management system this may be text, pictures and / or the actual menu structure and overall layout of your site.
At one extreme software such as Dreamweaver and Frontpage can be viewed as, and are used by some people, as content management systems. It gives them control over the content of their website. However, a major downside is the skill level required to utilise such software, limiting the number of people who can reliably use it.
At the other extreme there are solutions costing hundreds of thousands of pounds. Again there may be training issues here. However, a major practical problem is controlling what is probably a very large site.
The Control System sits somewhere in the middle. It doesn't require much training, and doesn't allow fundamental changes to be made that may break the whole website.
Content management systems fall into two categories. Some require special software to be installed on any computer of someone updating the website. Obviously this may increase licensing costs, and limits who can update the website from where. Others, including the Control System allow your website to be updated from practically most PCs worldwide. In practice this tends to mean home or office, although we have at least one client who does things while away on holiday!