A content management system (CMS) allows web site owners to add, edit and remove text, images and other media on a web page, without needing to know any web coding. There are many different CMSs available, from commercial versions to open source (free). The most popular CMS today is the open source platform WordPress.
WordPress is free to download and has a huge community of developers and designers constantly updating and supporting it. Whatever functionality you need for your site, there is a WordPress plug-in to achieve it.
WordPress started out as a blogging platform, used mainly to post articles and news events. It has become much more powerful than that and now supports full e-commerce solutions and social networking functions.
There are other open source CMSs available. The two other major competitors are Joomla! and Drupal. I choose WordPress because it has the most user-friendly admin section. It is important that the system is easy for clients to use.