CMS means Content Management system
It is a piece of software that allows the information or content of a website to be adapted via a simple interface (usually via a website, but this does not have to be compressed). A CMS is especially important for websites whose content needs regular customization, and the content in a fixed layout is presented to visitors. Most large companies use a CMS for their website nowadays. For example, a known variation of the CMS is the weblog. There are many different types, so something for everyone. Companies often opt for a larger (expensive) system, while SME's can easily use a free (open source) CMS. A good start is the site CMS matrix.