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The Curious Bag
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Double-layer DVDs hold more information than single layer DVDs because it is encoded on two separate layers instead of just one. They're more expensive than single-layer DVDs, but their additional storage capacity makes up for the added cost.
Data
Data is encoded on DVDs in a spiral of tiny pits encoded on the disc. Each pit is exactly the same size and width as the others.
Laser
The laser in the DVD player reads the pits as a binary code (essentially a series of 1s and 0s). String enough code together, and you have an entire movie or similar data file.
Second Layer
Single-layer DVDs just have one spiral of pits; in double-layer DVDs, a second spiral is encoded a little deeper in the disc.
Reading
In order to let the laser read the second layer of information, the material on which it is encoded is translucent. That lets the laser focus through the first layer when necessary.
Pause
You can sometimes see the effects of a double-layer DVD when watching a movie. The image will suddenly pause for a moment as the laser changes layers.
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