OK It's long but definitive. The whole story lies here:
DVD origianlly read only. But just as compact disc technology evolved so that users could record and erase and re-record data onto compact discs, the same is now true of DVDs. Loads of formats though DVD+R, DVD+RW, DVDRAM, DVD-R, DVD-RW, DVD-ROM
The crucial difference among the standards is based on which manufacturers adhere to which standards.
Similar to the old VHS/Beta tape wars when VCRs first hit the markets, different manufacturers support different standards.
DVD+R and DVD+RW formats are supported by Philips, Sony, Hewlett-Packard, Dell, Ricoh, Yamaha and others.
DVD+R is a recordable DVD format similar to CDR. A DVD+R can only record data once and then the data becomes permanent on the disc. The disc can not be recorded onto a second time. DVD+RW is a re-recordable format similar to CDRW. The data on a DVD+RW disc can be erased
and recorded over numerous times without damaging the medium.
DVDs created by a +R/+RW device can be read by most commercial DVD-ROM players.
DVD-R, DVD-RW and DVD-RAM are supported by Panasonic, Toshiba,
Apple Computer, Hitachi, NEC, Pioneer, Samsung and Sharp.
These formats are also supported by the DVD Forum. DVD-R is a recordable DVD format similar to CDR and DVD+R. A DVD-R can only record data once and then the data becomes permanent on the disc. The disc can not be recorded onto a second time. There also are two additional standards for DVD-R disks: DVD-RG for general use, and DVD-RA for authoring, which is used for mastering DVD video or data and is not typically available to the general public.
DVD-RW is a re-recordable format similar to CDRW or DVD+RW. The data on a DVD-RW disc can be erased and recorded over numerous times without damaging the medium. DVDs created by a -R/-RW device can be read by most commercial DVD-ROM players.
DVD-RAM discs can be recorded and erased repeatedly but are only compatible with devices manufactured by the companies that support the
DVD-RAM format. DVD-RAM discs are typically housed in cartridges.
DVDs you buy from shop with movies on are DVD-ROM. DVD-ROM was the first DVD standard to hit the market and is a read-only format. The video or game content is pressed onto the DVD once and the DVD
will run on any DVD-ROM-equipped device.