by 
The Curious Bag
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Dual-layer DVDs are able to hold more data than single-layer DVDs because they utilize twice as much surface space. Standard DVD have one layer of information, while dual-layer DVDs have two, stacked one on top of the other.
Data
Every DVD contains a small spiral of tiny divots that hold information. Dual-layer DVDs contain a second embedded just above the first one.
Lasers
A laser in a DVD player scans the spiral as the disc spins and translates the information. The spirals on dual-layer DVDs are encoded on translucent material, which allows the laser to focus through the top layer to the second.
Pause
When the laser switches layers on a dual-layer DVD, there is often a slight pause during replay. You can see this when watching a movie--the movie will freeze briefly during the shift.
Recording
When a dual-layer DVD is placed in a recorder, the laser looks for what is called an address in pre-groove signal, which tells the laser there are two layers and if information has been recorded on them. When recording, the laser starts on the inside hub of the first layer and moves outward, then shifts to the outside hub of the second layer and moves inward.
Space
Dual-layer DVDs can hold up to four hours of moving images, 2,000 mp3 songs or 17,000 jpeg images.
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