The parts of a system not immediately visable to a user.

Many WebSites use a PostRelationalDataBase (MySQL for example) as a BackEnd, generating some of the content and/or decoration dynamically from the database to serve to the user. The database is definitely part of the backend, as might be the glue that binds the database to the webserver, depending on your definition of BackEnd.

See also: FrontEnd, BackOffice