RAM = Random Access Memory
ROM = Read Only Memory

you have RAM in your PC as the content of the RAM can and does change, however a DVD is solid state and you cant change it hence a DVD is ROM

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 80% 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 89% 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.