DVD - Digital Video Disk or Digital Versatile Disk
  An improved version of the CD

  Red laser (.065 microns vs 0.78 microns for CD)
     Tighter spiral (0.74 micron gap vs 1.6 micron for CD)

     Smaller pits (0.4 microns vs 0.8 microns for CD)

     Thinner recording layer allows multiple layers.

  Provides for a 7 fold increase of capacity.

  Increased linear speed( 1.4MB/sec vs 150KB/sec for CD)


DVD capacity
  Allows up to four layers.
     Single sided, single layer - 4.7 GB (133 minutes -mpeg 2)

     Single sided, double layer - 9 GB

     Double sided, single layer - 9.4 GB

     Double sided, double layer - 18 GB
       Not implemented - cost of accurate of equipment not justified.
 

DVD 2 layer technologies
  For dual layering
     Bottom layer has a fully reflective base

     This is topped with a semi-reflective surface for the upper layer.

     Bits on lower layer are slightly larger to be easily recognized.
       This results in the not quite doubling of storage.

     Second layer has track read from outside in. This allows for a movie
     longer than 133 minutes to be read continuously with only a minor time
     delay for laser refocusing.


Data protocols
  Designed primarily for movies

  Dual sided and dual layer were competing protocols.

  There are at least 9 protocols (including laser disk)
    DVD, VCD, etc
 
  Allows read restrictions based on movie ratings.

  Coded for regions of the world. (5?)
    Used to prevent cheaper foreign releases from being sold here.

  Also, two general TV standards NTSC and PAL (France)