Lasers

• The size of a DVD is determined by the length of the spiral. The coding in the spiral is read by a laser, which is accurate enough to distinguish individual lines in the grooves.
  
  

Layers

• Information in a DVD may be divided into multiple layers and/or have data encoded on both sides, which can increase the storage capacity of the disc.
  
  

Hiccups

• Have you ever noticed a slight pause in the action when you watch a DVD? That's caused by the layers switching layers to continue reading the information.
  
  

Size

• A single-layer DVD can hold about 4.7 gigabytes worth of information. Double layers can hold a little less than twice that amount: 8.5 gigabytes.
  
  

Length

• A single layer of the average DVD (4.7 Gb) translates into about 2 hours worth of movie time. Double-layered DVDs can hold about 4 hours worth.