Digital Video Disks:

DVD stands for Digital Video Disks or Digital Versatile Disks. A DVD stores much more data than a CD-ROM. Its capacities are 4.7GB, 8.5GB, and 20GB etc. The  capacity depends on whether it is a single layer, double layer; single sided or double sided disk. DVD uses laser beam of shorter wavelength than CD-ROM uses and therefore more tracks are available. Working principles of DVD disks are same as those of a CD-ROM, CD-R or  CD-RW.  

The Speed of CD-ROM or DVD-ROM is given in terms of nX, where n is an integer. For example 32X. In case of CD, X=150 KB/s, so 32X=32x150=4.8 MB/s. In case of DVD, X=1.38 MB/s.