In the old days of the web most sites where constructed from static html pages, this mean that every update to the site, no matter how small, had to be done by the client's web designer. To enable clients to edit their websites themselves, programmers developments 'Content Management Systems' (CMS). Instead of storing the pages on the server as static html, the html pages are generated on demand using html which is stored in an database on the server. In addition to the client being able to maintain the site there are a number of other advantages of using a CMS. As the content is stored in a database separately from the pages layout template, changes can be made to the template (the common parts of the page) and those changes will affect all the pages that use that template.
There are a number of Off-the-shelf Content management Systems available both commercial and free / Open-Source. One of the most popular Open CMS' is Wordpress which, though particular suited to Blog-style sites can also be modified in vast number of ways to suite the specific requirements of the website being built
Frequently when specifying a website, a web development company propose the use of a CMS' that they own and development. If this is the case then the client is most likely going to be bound to this developer for all future developments as the client will probably not allowed to take the code to another Web Developer. This type of content management system may also have annual licensing fees associated with which will add to the overall costs. Unless there is a compelling argument otherwise, it is often best to opt for an Open-Source Content Management system with a large active development community to provide you with the best value for money, software reliability and portability.