CMS is web jargon meaning Content Management System. A CMS website design is one where you the client can update the content of the site yourself, without knowing any fancy code or owning extra software packages.
CMS site allow for quick access to content changes and allow your website to grow as needed.
1. If your site requires frequent updates to images or text content
2. If your site carries large amounts of content
3. If you want to update and maintain your site yourself
4. If you want other people to contribute to your site