Even though DVD and CD media share the
same physical size, DVDs offer a far greater storage capacity through
the use of a higher density data storing technique and through the use
of layering. DVD layering is the process in which the read laser of a
drive is able to focus at different "layers" inside the
disc.  A dual-layered disc has an equivalent of close to twice the
surface area as a regular disc and a dual-layered, double-sided disc
has almost four times the surface area on which to store data.
Currently, there are three types of DVDs and four physical formats.
The three DVD types are DVD-ROM, DVD-Video, and DVD-Audio. DVD-ROM is designed for storing computer files. DVD-Video is the format used by stand-alone DVD players for movies and extras. DVD-Audio is a newer format that includes multiple channel audio with many options. DVD physical formats define the structure of the disc and the
areas to which data is recorded. DVD 5 is a single sided, single
layer DVD with a storage capacity of up to 4.7 GB. DVD 9 is a
single sided, dual-layered DVD and it has a total capacity of 8.5
GB. DVD 10 is a double sided, single layer per side DVD with a
capacity of 9.4 GB. Finally, DVD 18 is a double sided, dual-layer
per side disc with a total capacity of 17 GB. Figure
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is a table that shows a comparison between the different DVD
formats.