DVD-ROM, as its name implies, is a "read-only memory" format. Your computer can read a disc's contents, but it cannot save data to one.
DVD-RAM (DVD random-access memory) and DVD-RW (DVD rewritable) are both rewritable mediums. Both DVD-RW and DVD-RAM are rewritable DVD standards that have been published by the DVD Forum, a group of more than 200 industry-leading corporations (
http://www.dvdforum.org/).
See technical document 42718 "SuperDrive: About Rewriteable DVD Discs" for additional discussion of DVD-R, DVD-RW, DVD-RAM, and DVD+RW.