Secondary storage means external storage. This storage device includes a magnetic disk, optical disk, magnetic tape and others. The most widely used external storage is as follows:

Logical Data Elements

If there is no systematic way to store and retrieve data, it is too difficult to get any information from an information system. Therefore, data resource should be organized in some logical manner. Data are logically organized into characters, fields, records, files, and database.

• Character: A character is the most elementary logical data element, whereas bit and byte are basic physical storage elements. A character consists of a single alphabetic letter, numeric digit, or special symbol. The character is equivalent to a byte.

• Field: The next higher level of data is the field. A field consists of a grouping of related characters. For example, a set of characters in a customer's name makes a name field. A field represents an attribute of an entity.

• Record: A record is a collection of related fields. The record represents a collection of attributes that describe an entity.

• File: A set of related records makes a file. For example, a customer file consists of many records of customers.

• Database: A database is an integrated collection of logically related files. A database combines several records previously stored in separated files.

File: In word processing, a file is a single document. In computer graphics, a file is a set of image descriptors for one picture. In a database, a file is a collection of records.