A disk, typically of plastic, on whose surface information is etched as a sequence of pits, which are read by a low-power laser beam. Optical disks can potentially store large amounts of information (a billion bytes or more).
Unlike floppy and hard disks, which use electromagnetism to encode data, optical disk systems use a laser to read and write data. Optical disks have very large storage capacity, but they are not as fast as hard disks. The most common form of optical disk that you will encounter is the CD-ROM. The most common variety of CD can store up to 650MB (megabytes) or 74 minutes of audio recording time. A single CD-ROM has the storage capacity of 700 floppy disks, enough memory to store about 300,000 text pages. CD-ROMs are particularly well-suited to information that requires large storage capacity. This includes color large software applications, graphics, sound, and especially video. DVD disks are also commercially available at this time and can be used to record up to 4.7GB of video data on a disk that is the same size/shape of a traditional CD. Mini CDs are available that generally store 21 minutes of audio, or 50MB of data on a small surface roughly the size of a business card.