Actually, there're three competing DVD recording standards:
(1) DVD-R and DVD-RW
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The first DVD recording format released and is supported by DVDForum.
DVD-R is a non-rewriteable format and is compatible with about 88% of all DVD Players and DVD-ROMs.
DVD-RW is a rewriteable format and is compatible with about 69% of all DVD Players and DVD-ROMs.
(2) DVD+R and DVD+RW
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Supported by the DVD+RW Alliance.
DVD+R is a non-rewritable format and is compatible with about 84% of all DVD Players and DVD-ROMs.
DVD+RW is a rewritable format and is compatible with about 70% of all DVD Players and DVD-ROMs.
(3) DVD-RAM
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Supported by DVDForum. DVD-RAM has the best recording features but it is not compatible with most DVD players and DVD-ROMs. Think more of it as a removable hard disk.
In addition, both DVD-RW and DVD+RW supports single side 4.7GB DVDs (called DVD-5) and double side 9.4GB DVDs (called DVD-10). However, DVD+RW has some better features than DVD-RW like CAV and CLV writing.
This site gives a more detailed discussion on the topic:
<A HREF="http://www.anandtech.com/guides/viewfaq.html?i=118" target="_new">http://www.anandtech.com/guides/viewfaq.html?i=118</A>