It is a database driven web application that enable your average web user to push content through administration panel without having to delve into editing code. Example Joomla
A content management system (CMS) is the collection of procedures used to manage work flow in a collaborative environment. These procedures can be manual or computer-based. The procedures are designed to: * Allow for a large number of people to contribute to and share stored data * Control access to data, based on user roles. User roles define what information each user can view or edit * Aid in easy storage and retrieval of data * Reduce repetitive duplicate input * Improve the ease of report writing * Improve communication between users In a CMS, data can be defined as nearly anything - documents, movies, pictures, phone numbers, scientific data, etc. CMSs are frequently used for storing, controlling, revising, semantically enriching, and publishing documentation.
In most context, CMS's are referring to a platform which will allow you to login to a "dashboard" and use a visual editor (similar to Microsoft Word) where you can edit a web page as if it were a .doc file. Some are better than others. The two most popular are WordPress and Joomla. It makes it much easier for non-coding people to actively maintain/update a website.