DVD-ROM is a non-volatile optical data storage medium similar to CD-ROM. Like CD-ROMs, they use a similar technology of microscopic pits on a reflective surface that are read by a laser beam. The difference is that DVD-ROM allows information to be stored on several layers on both sides of the disk (double-sided, dual-layer) giving greater information density. DVDs and CDs are almost indistinguishable to the naked eye (DVDs are a little thicker) but a DVD storea up to 17 GB (when double sided, dual layered) of data compared to the limit of about 700 MB on a CD (which is single-sided, single-layer). There exists compact discs which are dual sided, but these are very unusual.
There has been some dispute about the meaning of the name - either 'Digital Video Disc - Read Only Memory' or 'Digital Versatile Disc - Read Only Memory'. The latter is preferable, due to the range of information that can be stored on the disks. Common uses include distribution of computer software and films - a single DVD can hold an entire feature film and several extra features, ranging from scene selection to alternative endings, director's commentaries and documentaries about the subject matter of the film.
The use of DVD-ROM for selling prerecorded films has grown at a fast rate and is overtaking video cassettes faster than CDs overtook cassette tapes for audio.
Informative DVD-ROMs may contain links to webpages with additional information. To keep them up to date these are sometimes indirect: they link to webpages maintained by the producer of the DVD-ROM which contain the links to external webpages. E.g. a Galileo DVD links to http://www.galileo-dvd.de/a_dam/e/pc06.htm , which links to [1] , with framing.
