A direct access disk written and read by light. CD, CD-ROM, DVD-ROM and DVD-Video are read-only optical discs that are recorded at the time of manufacture and cannot be erased. CD-R, DVD-R, DVD+R, WORM and magneto-optic (in WORM mode) discs are write-once. They are recorded in the user's environment, but cannot be erased. CD-RW, DVD-RAM, DVD-RW, DVD+RW and MO discs are rewritable.

Optical discs have some advantages over magnetic disks. They have higher capacities as removable cartridges, and they are not subject to head crashes or corruption from stray magnetic fields. They also have a 30-year life and are less vulnerable to extremes of hot and cold.

Magneto-Optic (MO) and Phase Change
Rewritable discs use either magneto-optic (MO) or phase change technology. Used in libraries that hold multiple cartridges, magneto-optic (MO) discs are extremely robust. Phase change disks (CD-RW, DVD-RAM, etc.) are lower cost consumer-oriented products. See DVD, phase change disc, holographic storage, ISO 13346, multilevel optical disc and legality of optical storage. See also magnetic disk and magnetic tape.

  Writability  Optical Disc Types

  Read only    CD, CD-ROM, DVD-ROM, DVD-Video
  Write once   CR-R, DVD-R, DVD+R, WORM
  Rewritable   CD-RW, DVD-RAM, DVD-RW, DVD+RW, MO