A Content Management System or CMS is a system that allows users to easily manage and update website copy and content. There are literally hundreds of CMS's from which to choose. Some are open-source which means that they are grown and managed by an open community of developers and are free to use. Others are proprietary meaning they are owned, managed, supported and scaled by a specific company. Regardless of whether you decide to employ an open source or proprietary solution, there will be costs involved because web designers, developers and software engineers will still be needed to actually build your website within the chosen CMS.
Content Management Systems offer a range of functionality. Some are very simple and only allow the user to make simple copy edits. Some might allow copy edits as well as content uploads such as images and files. More robust systems enable the client to designate user groups and permission levels for each user group. Enterprise-level systems allow modules to be developed by software engineers, offer scalability and are highly customizable. A module might be something simple such as an image gallery or something quite complex like an inventory system. These modules can then be “plugged in” to a client's website.
If you are thinking about updating or overhauling your website, I recommend you research the possibility of incorporating a CMS. If you have questions about content management options, I'd be happy to help you find the answers!