Compact Disc - Read Only Memory. A CD-ROM is any compact disc which contains computer data. These discs can store huge amounts of data (up to 640 megabytes). If there is a large amount of data on a CD-ROM, then it is usually impractical to copy the data on to the hard disk; in this case, you must insert the disc whenever you want to use the data. The ROM simply means that you can not save information onto these discs. CD-ROM may also refer to the drive used to read these discs.