Acronym for Compact Disc - Read Only Memory
Note that because CDs were "invented" by Philips (European) and Sony (Japanese), they are "discs" and not the American "disks" (as in hard disk drive).
Standard discs hold 650MB or there abouts. The normal filesystem used on a data CDROM is iso9660, although most discs also use MicrosoftCorporation's Joliet extensions that allow some things like long filenames (although some characters that are valid on unix filesystems are not allowed). Another such extension is called Rock Ridge.
26 pages link to CDROM: