CD-ROM. View of a compact disc used for storing computer data. Discs like this are known as CD- ROMs (Compact Disc Read-Only Memory). Images, sound and computer data can all be stored on compact discs. This information is coded digitally as zeroes and ones (binary digits). A compact disk is made of plastic pressed with a series of fine depressions that represent digitised signals of coded information capable of being read by laser. The encoded layer is coated with a film of metal to reflect laser light. CDs have a large data storage capacity.