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DVD-R and DVD-RW
DVD-R/W was the first DVD recording format released that was compatible with standalone DVD Players.
DVD-R is a non-rewriteable format and it is compatible with about 89% of all DVD Players and DVD-ROMs.
DVD-RW is a rewriteable format and it is compatible with about 72% of all DVD Players and DVD-ROMs.
DVD-R/W supports single side 4.7 GB* DVDs(called DVD-5) and double side 9.4 GB* DVDs(called DVD-10).
These formats are supported by DVDForum.
DVD+R and DVD+RW
DVD+R/W has some better features than DVD-R/W such as lossless linking and both CAV and CLV writing.
DVD+R is a non-rewritable format and it is compatible with about 83% of all DVD Players and DVD-ROMs.
DVD+RW is a rewritable format and is compatible with about 72% of all DVD Players and DVD-ROMs.
DVD+R/W supports single side 4.7 GB* DVDs(called DVD-5) and double side 9.4 GB* DVDs(called DVD-10).
These formats are supported by the DVD+RW Alliance.
DVD-RAM
DVD-RAM has the best recording features but it is not compatible with most DVD-ROM drives and DVD-Video players. Think more of it as a removable hard disk. This format is supported by DVDForum.
Realistically, the differences between -R and +R are growing smaller and smaller. Both are going to be compatible with most any newer model stand alone DVD player and DVD-ROM drive. DVD-RAM is compatible with very few DVD-ROM drives and few (no?) stand alone players. This latter is by design, as DVD-RAM is basically for nothing more than data archiving/backups. If you are doing video and want DVDs to watch on your set top box, either -R or +R will be okay. Or go for a dual-format drive. If data archiving/back up is your sole purpose, go with DVD-RAM, as it is likley cheaper.
\Dan