The same kinds of technology that enable us to store music digitally on compact disks allows us to store other kinds of information on the same medium. Compact Disks essentially store numbers. They are inexpensive to create, and can hold large amounts of information. (600 Mb) CD-ROMs are frequently used to sell software which has become too large to fit on floppy disks. Unfortunately, a CD - ROM cannot be written to with typical home technology. The ROM part of CD-ROM refers to this characteristic. A CD - ROM is an input device because it can send information to the CPU, but the CPU cannot send information to it.
CD Drives are measured in comparison to the speed of music CDs. An audio CD always runs at a constant speed. A 2X CD drive is twice the speed of an audio CD player. 6X and 8x drives are available at this writing. Even these drives are not as fast as typical hard drives.
Erasable CD drives are becoming popular, because they hold as much information as a regular CD, but the user can store things to them as well as reading from them. Such devices will probably be much more prevelant as their price comes down and reliability improves.