A content management system is software that keeps track of every piece of content on your Web site, much like your local public library keeps track of books and stores them.

Content can be simple text, photos, music, video, documents, or just about anything you can think of. A major advantage of using a CMS is that it requires almost no technical skill or knowledge to manage. Since the CMS manages all your content, you don't have to.

In a CMS, data can be defined as almost anything - documents, movies, pictures, phone numbers, scientific data, etc.

CMS's are frequently used for storing, controlling, revising, semantically enriching, and publishing documentation. Content that is controlled is industry-specific. For example, entertainment content differs from the design documents for a fighter jet. There are various terms for systems (related processes) that do this. Examples are web content management, digital asset management, digital records management and electronic content management. Synchronization of intermediate steps, and collation into a final product are common goals of each.

You can read more about CMS at wikipedia.