A CD-ROM is a circular optical media used to store data. It it an abbreviation of "Compact Disk Read Only Memory"
Originally designed by Phillips/Marantz to store music only, they were quickly adopted by the computer industry which used them to store read-only computer data and they became popular as a means to distribute software. In comparison to the alternative of floppy disks and cassette tapes, they were far more resilient and inexpensive to mass-produce.
A standard-size (120mm) CD-ROM contains just over 650MB of data in total.