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 featured films, etc.) with other media (generally comics and original novels). This typically 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. A very popular example of this is fan fictions. Fan fictions are stories created by a fan base that portrays existing characters in new and different ways.
Two prominent examples of media franchises with an expanded 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.
Although there are some exceptions, Expanded Universe works are generally accepted as canon, or part of the 'official' storyline. Otherwise, they are generally seen as 'apocrypha'. In some cases, characters created for an Expanded Universe can become 'adopted' (featured) by the primary works associated with that franchise, and vice versa - one example being with the Star Wars expanded universe. With the Whoniverse it is less clear what is canon, it has even been claimed they could all be canon due to history being changed.