The term Expanded Universe (sometimes called an Extended Universe) is generally used to denote the 'extension' of a media franchise (i.e. a television show, series of feature films, etc.) with other media (generally comics and original novels). This typically simply involves new adventures for existing characters already developed within the franchise; however in some case entirely new characters and complex mythology are developed. This is not the same as an adaptation, which is a retelling of the same story, usually on a different medium. Nearly every media franchise with a committed fan base has some form of expanded universe.
Two prominent examples of media franchises with an extended universe are Star Wars and Star Trek - both have a wide range of original novels, comics, video games, and other media that add to the mythology of each universe in different ways. In both cases entirely new characters and situations have been developed that exist only within the Expanded Universe media.