yeah, shima is wrong. videohelp.com is definetly your best resourcefor a question like this.
here's the answer from the site:
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 93% of all DVD Players and most DVD-ROMs.
DVD-RW is a rewriteable format and it is compatible with about 79% of all DVD Players and most DVD-ROMs.
DVD-R/W supports single side 4.37 computer GB* DVDs(called DVD-5) and double sided 8.75 computer 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 88% of all DVD Players and most DVD-ROMs.
DVD+RW is a rewritable format and is compatible with about 79% of all DVD Players and most DVD-ROMs.
DVD+R/W supports single side 4.37 computer GB* DVDs(called DVD-5) and double side 8.75 computer GB* DVDs(called DVD-10).
These formats are supported by the DVD+RW Alliance.
As Canucks Fan said, the only important difference is that - is more compatible than +, but only by 5%.