Dual layer and double layer DVDs are the same thing and are also referred to as DVD-9 discs and have a capacity of 8.5GB.
In dual layer DVD-9 discs, two layers of standard DVD-5 are joined together with a transparent spacer and a thin reflector between the two. The bottom layer is read and written to in exactly the same manner as DVD-5. Reading and writing to the second layer is achieved by the laser focusing a fraction of a millimeter beyond the first recording layer.
Dual layer DVD discs are 12cm in diameter and can have the content data added through the production processes of either DVD duplication or DVD replication. Dual layer discs are not as common as the single layer format as the 4.7GB capacity of the single layer discs is usually sufficient for most business uses and dual layer discs are also a lot more expensive.
Will my DVD drive be able to record dual layer DVDs?
Typically only the newer DVD recorder drives will be able to write to dual layer or double layer discs. You should look on the face of the drive tray for the logo shown to the right. The extra rectangular border in the top right and the 'DL' are the key differences to mark these drives out from standard DVD writer drives that only record to single layer discs.
DVDs have much higher storage capacities than CDs by using smaller pit sizes and narrower track pitches in the spiral groove that runs around the discs. These smaller pits and grooves require a different wavelength of laser light (650nm) to that used in CD drives (780nm) and therefore DVD discs cannot be read in standard CD drives.