When people first become webmasters, they usually get a little overwhelmed. After all, there is a TON of things to know about developing and running websites, and you couldn't possibly absorb all the information without spending at least a few years doing it. Fortunately there are things out there specifically designed to help newbie webmasters, and professionals who don't have time to do everything manually. Some of the most helpful things out there are content management systems, or CMS'.
Content management systems do exactly what they sound like they should do: manage content. Content is a pretty general term used by most webmasters to describe anything they uploaded to their website - articles, blog posts, pictures, videos, etc. The design of the website is not usually considered content. With a CMS, you'll be able to upload content without too much expertise. Basically, if you can check your email, you can upload content with a CMS. Operating a website without a CMS is much more complex, and requires a good deal of web development knowledge and website programming.
E-Commerce and Analytics
There are basic CMS' that allow you to run basic websites with ease, and then there are CMS' designed specifically for E-Commerce. E-Commerce is simply the exchange of products for money over the Internet. Certain CMS' can only be used for E-Commerce sites, while others simply have E-Commerce plugins or add-ons that allow you to do basically the same thing as the full-fledged E-Commerce CMS' do. Most CMS' also have Analytics capability. Depending on the program/plug-in, you will be able to track number of visitors, where they came from, how long they stayed, which pages drew the most attention, etc. More advanced Analytics programs can do a lot more than that, but you usually have to pay for them.