CMS is an abbreviation for "Content Management System" and is in short gives an authorised user the ability to change content on their own website.
Such things as "Blogs" use content management systems to add and edit content and can be very useful for those sites that need to add or change content on a regular basis. It gives the customer more control of their website and also keeps costs down and increases productivity.
A classic example for the need for content management is ecommerce. Can you imagine having to call an agency just to get them to change a price; it's simply not practical in most cases. With the use of a content management system the user can quickly add or edit a price, add a new product or simply alter some text.
Nearly all of our ecommerce products are built with content management. We also build bespoke CMS systems for smaller websites where individual pages have to be changed on a regular basis, news pages for example.