Compact discs (CD), introduced in the early 1980s, are 12 centimeter wide, one millimeter
thick discs hold nearly 700 megabytes of data, or more than an hour of music. They ship in three formats: CD-ROM (read-only memory) discs that come filled with data and can be read from but not copied to; CD-R discs that can be written to once; and CD-RW discs that can be written to multiple
times. Most computers ship with CD drives capable of recording (”burning”) CDs.