CD-ROM an acronym of "Compact Disc Read-only memory") is a pre-pressed compact disc that contains data accessible to, but not writable by, a computer for data storage and music playback. The 1985 “Yellow Book” standard developed by Sony and Philips adapted the format to hold any form of binary data.
DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than compact discs while having the same dimensions.
Pre-recorded DVDs are mass-produced using molding machines that physically stamp data onto the DVD. Such discs are known as DVD-ROM, because data can only be read and not written nor erased.
3. Combo Drive:
A Combo drive is a type of optical drive that combines CD-R/CD-RW recording capability with the ability to read (but not write) DVD media. The term is used almost exclusively by Apple Inc. The device was created as a mid-range option between a CD burner and a DVD readable, which at the time the Combo drive was introduced was generally an expensive option costing in excess of US$300 a unit.