A WORM device is a type of
backing store. WORM stands for
Write Once, Read Many. An example of a WORM device is a CD-ROM. Once it's burnt, its then read over and over again. Punch-cards were an example of this in the olden days. In contrast, a
hard disk is not a WORM device because it is constantly re-written and changed.
WORM devices tend to be quite cheap, because no-one wants to pay a lot for a device they can only write to once. This especially applies to CD-ROMs, which are so cheap because the process of burning them is a one-way ride. CD-RWs are more expensive because they carry the capability to be written more times.