Information in computer-based filing systems is stored in data files.

A file is a collection of related records.

Related records means that each record in a file will contain the same sort of information as all the other records.

A record must have at least one field. A field contains one individual item of data.

The arrow below is pointing to the fields, there are four fields in each record of this file 

ISBN is the key field. This is the field that has a different value in every record. It is used to distinguish one record from another. A key field uniquely identifies an individual record.