Hvad er forskellen på de forskellige formater?

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 90% of all DVD Players and most DVD-ROMs.
DVD-RW is a rewriteable format and it is compatible with about 73% of all DVD Players and most 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 85% of all DVD Players and most DVD-ROMs.
DVD+RW is a rewritable format and is compatible with about 73% of all DVD Players and most 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. DVD-RAM is usually used in some DVD Recorders.

DVD+R/+RW - DVD Writhe + ReWriteable
DVD-RW - rerewriteable DVD
DVD Rom Alm DVD

lidt ekstra info:

DVD-5
DVD-5 is a single sided single layer DVD that stores up to about 4 707 000 000 bytes and that is 4.38 GigaBytes where 1 kilobyte is 1024 bytes(4 707 000 000B / 1024 = about 4 596 679KB / 1024 = about 4489MB / 1024 = about 4.38GB) . Video DVD, DVD-R/W and DVD+R/W supports this format. Often referred to as "single sided, single layer".

DVD-9
DVD-9 is a single sided dual layer DVD which can fit up to 7.95 computer GB which many commercial video DVDs are using today (a DVD-9 is basicly two pressed plastic DVD-5s pressed together, they are not burned). Video DVD supports this format but DVD-R/W and DVD+R/W does not support this format.

Det var hurtigt svar, jeg venter lidt med point, jeg vil lige høre om der er nogen der kan fortælle om der er noget der er bedre end andet, evt. anbefale hvad jeg skal bruge!

So what should I buy then?

This is a very difficult question. The choice largely depends on one's usage environment and preferences.

DVD-RAM discs cannot be used in most standalone DVD players and DVD-ROM drives, and this has historically made this format less attractive for the average home user. However, the integrated OS-level drag-and-drop read/write data support of DVD-RAM makes it very attractive for some users, especially now that most current DVD-RAM burners also support DVD-R burning for video applications. DVD-RAM capable PC drives will also be ideal for those whom already own DVD-RAM based camcorders or standalone DVD-RAM recorders.

DVD-R/DVD-RW drives currently have the highest market penetration, both with PCs and Macs. As previously mentioned, DVD-R enjoys high compatibility with standalone DVD players. In addition, the low cost of DVD-RW is attractive to many for backup purposes (with the use of third party software and/or drivers). Furthermore, most standalone DVD players will read DVD-RW as well, although the compatibility rate is significantly lower than with DVD-R.

+R and +RW are beginning to gain market share, and these discs appear to have similar compatibility on standalone DVD players as compared to DVD-R are DVD-RW, respectively. Current functionality with these drives is similar to DVD-R and DVD-RW drives, both for data and for video applications, and recent experience suggests that +R/+RW drives are equally reliable.

And of course, as mentioned earlier, newer multiformat drives exist (albeit often at higher cost).