For storing data, the disc is divided into tracks and tiny pits in the tracks correspond to the zeros and ones of the digital information. The pits are read by a laser and played back.

On a DVD, track pitch is about half that on a CD, so tracks and pits are finer. As a result, high-density data storage is achieved. To read the data a red laser is used. It has a much shorter wavelength than the laser used for reading CDs, and can read much more finely.

By focusing the laser light through a large-aperture lens, a narrower focus is obtained and high-density data reading achieved.

However, because the laser is greatly refracted by the use of a large-aperture lens, it is difficult to obtain an accurate focus if the disc is not precisely horizontal in relation to the laser. This difficulty can be avoided by making the disc thinner. That is why the thickness of a DVD is just 0.6mm, half that of a CD, and the back-to-back bonded structure is adopted.