A data dictionary contains information about the data items in a database, including the entity or table name, attribute or field name, data type, size, whether it is required, and any validation rules. For example, a school data dictionary would specify that the Class table has attributes like ClassID which is the primary key, Classroom which is a number that must be between 1 and 10, and MainBuilding which is a yes/no field. It also describes pupil attributes like CandidateNum which is the primary key consisting of 7 numeric characters and ClassID which is a foreign key that must match an existing ClassID.