A content management system is the part of your website which you can log in to and make changes to pages and products etc. You may also be able to make small design changes and add new pages and new buttons in the top navigation. You can choose to use an off the shelf content management system or a bespoke one, which will be made to include all the functionality you need. There are pros and cons to both types of content management systems and deciding the one to choose depends entirely on the website development company and the functionality you require. Some off the shelf content management systems simply do not give you the ability to change everything you need to and therefore you will need to get a bespoke system created. If you are just looking for a basic website that will offer information or that is a simple ecommerce site, then you will probably find an off the shelf content management system will do the job.
Most bespoke systems can also be added to at a later date if you want to add further functionality in to it. The developer will be able to amend the content management system and add in the elements you require. This is handy if you are looking to grow your business quite quickly and need to ensure your website will be able to keep up.