CD stands for compact disc which is a general term for all formats of CD media. CD formats available on the market now include CD Audio, CD-ROM, CD-ROMXA, VideoCD, CD-I and others. The CD audio and CD/ROM standards were originally developed as a read-only medium. However, mechanical and optical technologies were rapidly developed to permit a writable CD format. These writable formats are variously called CD-R (CD-recordable), Write-Once Read-Many (WORM) or CD-WO. CDs are 4-3/4'' in diameter. CDs are faster and more accurate than magnetic tape for data storage. Compact discs are made from a 1.2 mm thick disc of polycarbonate plastic coated with a much thinner layer of super purity aluminium (or rarely, gold, used for its data longevity, such as in some limited-edition audiophile CDs) layer which is protected by a film of lacquer. The lacquer can be printed with a label. Compact disc players play back digital sound recordings from a compact disc, and digital sound recordings have a higher sound quality than do analog signal recordings. Also, compact disc players provide for fast movement to the previous or next sound track. A CD audio disk holds approximately 74 minutes of stereo music recorded with 16-bit resolution -- and incorporates a number of error reduction, detection and correction techniques.
References: Professor Kelin J. Kuhn "Audio Compact Disk", Wikipedia article "Compact disc"