A database is a logically organised collection of related data, generally accessed by a set of programs known as a Database Management System (DBMS), which oversees the creation and use of the database and controls access to the data. The organisation of a database obviates the need to duplicate information to meet the various requirements of different groups of users, and ensures that the data always remains consistent. A large database requires extensive storage facilities. In some organisations and services, databases can be accessed over networks from microcomputers or as videotex. 'Relational' databases and hypertext techniques include extensive and complex cross-reference facilities so that information on related items may be retrieved. Many database programs have been designed to run on micro-computers. Some of these contain computer languages that enable users to change the operation of the database to suit their requirements.