There are other types of optical disk media in addition to CDs, DVDs, HD DVDs, and BLU RAY discs. These other optical discs have different sizes than the standard CD size of 12 cm or 4.75 inches and are different in structure and form than the CDs, DVDs, etc.
The Laserdisc can also be referred to by a number of other names, such as the Laserdisk, Laser Video Disk, Laser Video Disc, video disc, LaserVison disc, LV disc, or LD disc. This was the first type of read-only optical disc and was introduced by Philips in the 1970s. The Laser disk was manufactured in three sizes with the most common or standard size being 30 cm or 12 inches. It was commonly used for movies and a variety of other video applications such as music videos and training applications.
WORM is an acronym for Write Once Read Many times and was first introduced in the 1980s. These discs are similar to the recordable CD, DVD, and Blu ray formats in that once the disc is full you can no longer record to the disc or erase the information on it and then reuse it. The WORM optical disc comes in a variety of different sizes with the most popular sizes being the 12 inch and 5.25 inch formats. A significant difference with other optical disk media is that these discs are not used bare but rather in a special plastic cartridge. Because of this and other technological differences, these discs require their own drive and cannot be read in standard CD, DVD, or BLU RAY drives.
Magneto-Optical or MO
The magneto-optical disc or MO disc is a non-CD size erasable optical disk format that was introduced late in the 1980s. Like the WORM disks, these discs are housed and used in plastic cartridges and cannot be played in standard optical disc drives. The common sizes for MO optical media are 5.25 inches, 3.5 inches, and 2.5 inches (the Sony MiniDisc).
UDO and PDD
These are write-once or erasable cartridge optical disc formats similar to MO discs, but are read with a blue laser rather than a red laser. This means that these disks provide much higher storage capacity than the magneto optical discs. UDO stands for Ultra Density Optical and Plasmon is the major backer and manufacturer. PDD is short for Professional Disc for Data and is supported primarily by Sony. The actual disc size for both UDO and PDD is 5.25 inches.