1.Variety of compact disc
1.1 According to the way of coding, include: CD,MP3,VCD,SVCD,CD-ROM and so on.
1.2 According to its longevity, include: century treasure: Gold disc, economic treasure: Silver disc.
1.3 According to their physical appearance, include: common disc: 120mm, mini disc: 80mm, and other special disc.

2.Variety of DVD
2.1 According to the way of coding, include: DVD, DVD-ROM and so on.
2.2 According to its structure, include four types: DVD5: one side with single data layer; DVD9:one side with double data layers; DVD10: two sides with single data layer; DVD18: two sides with double data layers.

3.Capacity of the discs:
The capacity of a compact disc is about 650 MB, equal to the capacity of 450 pieces 3.5inches floppy disk. The capacity of a DVD5 is 4.7GB, equal to 7 pieces compact disc. The capacity of a DVD9 IS 8.5GB, equal to 11 pieces compact disc.

4.Physical format
A basic CD-ROM disc is 120mm in diameter and 1.2mm thick. It is formed from three layers: a back layer of clear polycarbonate plastic, a thin sheet of aluminum and a lacquer coating to protect the disc from scratches and dust. The most common format of CD-ROM holds approximately 800 megabytes of data which is approximately equal to 80 minutes of audio. During the traditional mastering process, a polycarbonate blank is stamped with millions of tiny indentations called pits -molded is a spiral from the center of the disk outwards-, which is then coated, with a thin layer of aluminum, giving the disc its characteristic silver color. Unraveled and laid in a straight line the spiral of data would stretch four miles!