DVD (also known as "Digital Versatile Disc" or "Digital Video Disc" - see Etymology) is a popular optical disc storage media format. Its main uses are video and data storage. Most DVDs are of the same dimensions as compact discs (CDs) but store more than six times as much data.
The DVD-ROM specification supports disks with capacities of from 4.7GB to 17GB and access rates of 600 KBps to 1.3 MBps. One of the best features of DVD-ROM drives is that they are backward-compatible with CD-ROMs. This means that DVD-ROM players can play old CD-ROMs, CD-I disks, and video CDs, as well as new DVD-ROMs. Newer DVD players can also read CD-R disks.
A laser beam in the DVD player tracks the beam as the disc spins, while a special device reads the intensity of the reflection as it bounces off the pits and lands. The reflective variance gets translated to bits of data which form bytes. Hence, DVDs, including the DVD-ROM, can vary in capacity as follows:
* Single-sided single-layer disc — 4.38 GB
* Single-sided double-layer disc — 7.95 GB
* Double-sided single-layer disc — 8.75 GB
* Double-sided double-layer disc — 15.9 GB
DVD-ROM is an input device because it holds information that is readable. There is also re-writtable cd's which allow you to add or remove information making it an input device.