A storage structure is a file arrangement providing a way to access data in a database table.

Keyed storage structures provide fast access to a particular row or set of rows in a database table.

A key is the field or fields that the table is indexed on. Specifying this key gives you quick access to the rows you are looking for.

An index contains the contents of the key fields. (In X100, a primary index is a table organized as an index.)

A secondary index allows you to specify an additional key.