In a CMS, data can be defined as nearly anything: documents, movies, pictures, phone numbers, scientific data, and so forth. CMSs are frequently used for storing, controlling, revising, semantically enriching, and publishing documentation. Serving as a central repository, the CMS increases the version level of new updates to an already existing file. Version control is one of the primary advantages of a CMS.

A web content management system (WCMS) is a software system which provides website authoring, collaboration and administration tools designed to allow users with little knowledge of web programming languages or markup languages to create and manage the site's content with relative ease. A rich WCMS provides the foundation for collaboration, offering users the ability to manage documents and output for multiple author editing and participation.

Most systems use a database to store content, metadata, or artifacts that might be needed by the system. Content is frequently, but not universally, stored as XML, to facilitate, reuse, and enable flexible presentation options.

A presentation layer displays the content to Web-site visitors based on a set of templates. The templates are sometimes XSLT files. Most systems use server side caching boosting performance. This works best when the WCMS is not changed often but visits happen on a regular basis.

Administration is typically done through browser-based interfaces, but some systems require the use of a fat client. Unlike Web-site builders, a WCMS allows non-technical users to make changes to a website with little training. A WCMS typically requires an experienced coder to set up and add features, but is primarily a Web-site maintenance tool for non-technical administrators. Information sourced from Wikipedia.

Further information can be viewed at Wikipedia.
http://en.wikipedia.org/wiki/Content_management_system & http://en.wikipedia.org/wiki/Web_content_management_system