A file still if it is stored on a magnetic disk or CD-ROM disk may perhaps have a sequential file organization. These records in this type of sequential file (even though the file is on a medium that allows for direct access) may only be retrieved sequentially.
Alternatively, a file on disk is able to support random access to records if its file organization is either Direct or Indexed. With Indexed and Direct file organization random record access is achieved by means of a "key field".
With Direct file organization records are allocate relative record numbers based on a mathematical formula known as a hashing algorithm. For instance you input a Student ID Number a mathematical formula is applied to it, and the resultant value is the value that points to the storage location on disk where the record is able to be found.
An Indexed file comprises a table that relates key values (for example Student ID Number) to storage locations of the corresponding records. This table is called as the index. It is simply like the index of a book where the key value (topic) has a pointer to the storage location (page number) where the information is stored.
File Organization Supported Record Access
Direct Sequential or Random
Indexed Sequential or Random