Content mean something contained, as in a receptacle, Often used in plural: the contents of my desk drawer, the contents of an aerosol can, and also material, including text , images and video.
Web content is the textual, visual or aural content that is encountered as part of the user experience on websites. It may include, among other things: text, images, sounds, videos and animations.  A content management system (CMS) is the collection of procedures used to manage work flow in a collaborative environment, and a software that is used to create and manage the content for a Web site. It provides for the storage, maintenance and retrieval of HTML and XML documents and all related image, audio and video files.
Typically dependent on a particular database, which may be part of the package or available separately as in the case of open source versions, a Content Management System (CMS) may provide all the programs necessary for Web site development.