ROM

ROM is "built-in" computer memory containing data that normally can only be read, not
written to. ROM contains the programming that allows your computer to be "booted
up" or regenerated each time you turn it on. Unlike a computer's random access memory
(RAM), the data in ROM is not lost when the computer power is turned off.
The ROM is sustained by a small long-life battery in your computer.
Hard disk
Hard disk is a computer storage device which saves and retrieves the data when
required. Its capacity is much greater than the computer memory (RAM, ROM). Data on
hard disk is stored and retrieved from electromagnetically charged surface.
Today we can save huge amount of data on a single hard disk. Now hard disks can
contain several billion bytes.
Floppy disk
A diskette is a random access, removable data storage medium that can be used with
personal computers. The term usually refers to the magnetic medium housed in a rigid
plastic cartridge measuring 3.5 inches square and about 2 millimeters thick. Also called a
"3.5-inch diskette," it can store up to 1.44 megabytes (MB) of data.
Tape
In computers, tape is an external storage medium, usually both readable and writable,
can store data in the form of electromagnetic charges that can be read and also erased. A
tape drive is the device that positions, writes from, and reads to the tape.
CD
A compact disc [sometimes spelled disk] (CD) is a small, portable, round medium for
electronically recording, storing, and playing back audio, video, text, and other
information in digital form.
DVD
DVD (digital versatile disc) is an optical disc technology that is expected to rapidly
replace the CD-ROM disc (as well as the audio compact disc) over the next few years.
The digital versatile disc (DVD) holds 4.7 gigabyte of information on one of its two
sides, or enough for a 133-minute movie.
4.14 Classifying Memory/Storage
Electronic (RAM, ROM), magnetic (HD, FD, Tape), optical (CD, DVD)
Volatile (RAM), non-volatile (HD)
Direct access (RAM, HD), serial access (Tape)
Read/write (HD, RAM), read-only (CD)